JP3931876B2 - Magnetoresistive device and manufacturing method thereof - Google Patents

Description

  The present invention relates to a magnetoresistive device and a manufacturing method thereof, and more particularly to a magnetoresistive device including a magnetic tunnel junction (MTJ) exhibiting a tunnel magnetoresistive effect (TMR effect) and a manufacturing method thereof.

  A magnetic tunnel junction composed of two ferromagnetic layers and a tunnel barrier layer (tunnel insulating layer) sandwiched between these ferromagnetic layers depends on the relative direction of magnetization of the ferromagnetic layers. , Its resistance changes greatly. Such a phenomenon is called a tunnel magnetoresistance effect. By detecting the resistance of the magnetic tunnel junction, it is possible to determine the magnetization direction of the ferromagnetic layer. Utilizing such a property of the magnetic tunnel junction, the magnetoresistive device including the magnetic tunnel junction is applied to a magnetic random access memory (MRAM) that holds data in a nonvolatile manner and a magnetic read head for a hard disk.

  A magnetoresistive element including a magnetic tunnel junction typically includes a fixed ferromagnetic layer, a free ferromagnetic layer, and a tunnel insulating layer interposed between the fixed ferromagnetic layer and the free ferromagnetic layer. Consists of. The fixed ferromagnetic layer has a spontaneous magnetization whose direction is fixed, and the free ferromagnetic layer has a spontaneous magnetization whose direction can be reversed.

  In order to firmly fix the direction of spontaneous magnetization, the pinned ferromagnetic layer is often formed bonded to the antiferromagnetic layer. The exchange interaction that the antiferromagnetic layer gives to the pinned ferromagnetic layer firmly fixes the spontaneous magnetization of the pinned ferromagnetic layer. The antiferromagnetic layer is generally formed of an antiferromagnetic material containing Mn such as IrMn and PtMn.

  Furthermore, the free ferromagnetic layer is often composed of a hard ferromagnetic layer made of a ferromagnetic material having a high spin polarizability and a soft ferromagnetic layer made of a soft ferromagnetic material. Such a free ferromagnetic layer structure reduces the reversal field (or coercive field or coercive force) of the spontaneous magnetization of the free ferromagnetic layer while increasing the magnetoresistance change rate (MR ratio) of the magnetic tunnel junction. Make it possible. The hard ferromagnetic layer is generally formed of a ferromagnetic material including Co such as Co or CoFe, and the soft ferromagnetic layer is generally formed of a ferromagnetic material including Ni such as NiFe.

One of the challenges of magnetoresistive elements is thermal stability. When a high temperature is applied to the magnetoresistive element, mutual diffusion occurs between layers constituting the magnetoresistive element. This mutual diffusion degrades the characteristics of the magnetoresistive element, particularly the MR ratio. For example, Patent Document 1 points out the problem of mutual diffusion between a hard ferromagnetic layer and a soft ferromagnetic layer. A particular problem is that Ni contained in the soft ferromagnetic layer diffuses into the hard ferromagnetic layer. The diffusion of Ni into the hard ferromagnetic layer deteriorates the characteristics of the magnetoresistive element. Patent Document 1 discloses that an oxide film or a nitride film that prevents mutual diffusion is provided between a hard ferromagnetic layer and a soft ferromagnetic layer. Patent Document 2 points out the problem that Mn diffuses from the antiferromagnetic material containing Mn into the pinned ferromagnetic layer. Patent Document 2 discloses that a fixed ferromagnetic layer is formed of two ferromagnetic layers and an insulating layer or an amorphous magnetic layer interposed between the ferromagnetic layers to diffuse Mn into the fixed ferromagnetic layer. A technique for preventing this is disclosed. Patent Document 3 discloses a technique for improving the thermal stability of a magnetoresistive element by inserting an oxide magnetic layer into a fixed ferromagnetic layer and a free ferromagnetic layer. Patent Document 4 points out the problem of interdiffusion between the free ferromagnetic layer and the buffer layer formed under the free ferromagnetic layer in order to improve the crystallinity of the free ferromagnetic layer. In Patent Document 4, an atomic diffusion barrier layer made of oxide, nitride, carbide, boride, fluoride, or the like is formed between a free ferromagnetic layer and a buffer layer to suppress the buffer layer and thermal diffusion. , Discloses technology to improve thermal stability.
JP 2000-20922 A JP 2002-158381 A JP 2001-237471 A Japanese Patent Laid-Open No. 10-65232

  Another problem with magnetoresistive elements is the reduction of the reversal field of the free ferromagnetic layer. In a free ferromagnetic layer formed of a laminated structure of a hard ferromagnetic layer and a soft ferromagnetic layer, the reversal magnetic field may not be sufficiently reduced in order to secure a large MR ratio.

Reduction of the switching field of the free ferromagnetic layer can be achieved by reducing the product of magnetization M _s and film thickness t of the free ferromagnetic layer (hereinafter referred to as “product M _s × t”). The following is an explanation of the reason. When the free ferromagnetic layer has uniaxial anisotropy, the switching magnetic field of the free ferromagnetic layer is determined by the total anisotropic magnetic field of the free ferromagnetic layer. In a magnetoresistive element having a submicron order size, the anisotropic magnetic field due to the shape anisotropy of the free ferromagnetic layer occupies most of the total anisotropic magnetic field of the free ferromagnetic layer. Thus, the switching field of the free ferromagnetic layer can be approximated to the anisotropy field (anisotropy field) H _a due to the shape anisotropy. In this case, the anisotropic magnetic field Ha is expressed by the following formula (1):
H _a = 4πM _s (N _x −N _y ), (1)
It is represented by Here, M _s is the saturation magnetization of the free ferromagnetic layer, and N _x and N _y are the demagnetizing factor in the short side direction and the demagnetizing factor in the long side direction of the magnetoresistive element, respectively. When the thickness of the free ferromagnetic layer is constant, N _x -N _y as long sides and the ratio of the length of the short side of the magneto-resistive element (aspect ratio) increases will increase and thus, the shape anisotropy anisotropic magnetic field H _a that also due to an increase. Furthermore, the free demagnetizing factor with a decrease in size of the ferromagnetic layers is to increase, the free size of the ferromagnetic layer is reduced demagnetizing field and anisotropy field H _a increases. If the aspect ratio is fixed, the anisotropy field H _a is represented by the following formula (2):
H _a -4πM _s × t / W, (2)
Is approximated by Here, W is the length of the short side of the free ferromagnetic layer, and t is the thickness of the free ferromagnetic layer. Equation (2) shows that the switching field of the free ferromagnetic layer can be reduced by reducing M _s × t. In general, by reducing the film thickness t of the free ferromagnetic layer, M _s × t can be reduced, thereby reducing the switching magnetic field of the free ferromagnetic layer.

  The inventor has found that the interdiffusion generated between the magnetoresistive element and the conductor that electrically connects the magnetoresistive element to another element adversely affects the characteristics of the magnetoresistive element when heat treatment is performed. I found out. In order for the magnetoresistive element to function, it is necessary to electrically connect the magnetoresistive element to another element (for example, a transistor). For this reason, the magnetoresistive element is connected to a conductor (for example, via and wiring) electrically connected to another element. Such a conductor is generally formed of Al, Cu, W, and TiN as in other semiconductor integrated circuits. The Ta, Ru, Zr, and Mo layers may be used as conductors that electrically connect the magnetoresistive element and other elements. As the influence of mutual diffusion between the conductor that electrically connects the magnetoresistive element to another element and the magnetoresistive element, there are the following three.

  First, when the material constituting the conductor diffuses into the magnetic tunnel junction, the MR ratio of the magnetic tunnel junction deteriorates. In particular, when the conductor contains Al, since Al diffuses at a relatively low temperature, the problem of diffusion of Al into the magnetic tunnel junction is serious. In addition, the diffusion of Mn of the antiferromagnetic layer and Ni of the soft ferromagnetic layer into the tunnel barrier layer is further enhanced by diffusion of the material constituting the conductor, particularly Al, into the antiferromagnetic layer and the soft ferromagnetic layer. In order to be promoted, the problem of diffusion of the material constituting the conductor into the magnetoresistive element is serious.

  Second, if the material constituting the magnetoresistive element diffuses into a conductor that electrically connects the magnetoresistive element and another element, the resistance of the conductor increases. An increase in the resistivity of the conductor deteriorates the SN ratio for detecting the resistance of the magnetic tunnel junction. In particular, Mn contained in the antiferromagnetic material and Ni contained in the soft ferromagnetic layer diffuse at a relatively low temperature, so the problem of increased resistance due to diffusion of Mn and Ni is serious.

Third, the diffusion of the material constituting the free ferromagnetic layer into the conductor that electrically connects the free ferromagnetic layer to other elements by heat treatment reduces the thickness t of the free ferromagnetic material. This makes it difficult to reduce the reversal magnetic field. This is because when the free ferromagnetic layer has a small thickness t and diffusion due to heat treatment occurs, the characteristics of the free ferromagnetic layer greatly fluctuate and the operation reliability of the magnetoresistive element is impaired. FIG. 26 is a graph showing the effect of heat treatment on 4πM _s × t of a free ferromagnetic layer with a thin film thickness t. The properties of the free ferromagnetic layer are measured under the following conditions: The measured sample structure is
Substrate / Ta (10 nm) / AlOx / Ni ₈₁ Fe ₁₉ / Ta (10 nm)
It is. The AlOx film is formed by oxidizing a 1.5 nm AL film. The thickness of the Ni ₈₁ Fe ₁₉ film is selected from 3.0 nm, 2.6 nm, and 2.2 nm. The Ni ₈₁ Fe ₁₉ film is formed by sputtering. The heat treatment temperature is 250 to 400 ° C., and the time is 30 minutes. The magnetization M _s is measured by a vibrating magnetometer.

As shown in FIG. 26, when the film thickness t of the Ni ₈₁ Fe ₁₉ film is 3.0 nm or less, 4πM _s × t changes drastically by the heat treatment, and further, the decrease in the film thickness t and the processing temperature For the increase, 4πM _s × t decreases rapidly. These samples have poor reproducibility. Thus, when the film thickness t is 3 nm, 4πM _s × t of the free ferromagnetic layer becomes unstable with respect to the heat treatment. Such instability prevents the film thickness t of the free ferromagnet from being reduced.

  It is desired to provide a technique for suppressing mutual diffusion between a conductor that electrically connects a magnetoresistive element and another element and the magnetoresistive element.

An object of the present invention is to provide a technique for further improving the thermal stability of a magnetoresistive element.
Another object of the present invention is to prevent interdiffusion between conductors (eg, vias and wiring) that electrically connect the magnetoresistive element to other elements and the layers constituting the magnetoresistive element, The object is to provide a technique for further improving the thermal stability of the magnetoresistive element.
Still another object of the present invention is to disperse a material contained in a layer constituting the magnetoresistive element, in particular, Ni, Mn, into a conductor that electrically connects the magnetoresistive element to another element. An object of the present invention is to provide a technique for preventing a phenomenon in which resistivity increases.
Still another object of the present invention is to prevent a phenomenon in which the characteristics of a magnetoresistive element deteriorate due to diffusion of a material contained in a conductor electrically connecting the magnetoresistive element to another element into the magnetoresistive element. To provide technology.
Still another object of the present invention is to maintain Mn and Ni contained in the magnetoresistive element in the tunnel of the magnetoresistive element while maintaining magnetic coupling and electrical coupling in the fixed ferromagnetic layer and the free ferromagnetic layer of the magnetoresistive element. An object of the present invention is to provide a technique for preventing a phenomenon in which the characteristics of a magnetoresistive element deteriorate due to diffusion into an insulating layer.
Still another object of the present invention is to provide a technique that makes it possible to reduce the reversal magnetic field by thinning the free ferromagnetic layer.
Still another object of the present invention is to provide a technique that prevents the diffusion of the material constituting the free ferromagnetic layer and makes it possible to reduce the thickness of the free ferromagnetic layer and reduce the switching field.
Still another object of the present invention is to reduce the reversal magnetic field while maintaining the squareness of the magnetoresistance curve of the free ferromagnetic layer and to suppress the variation of the reversal magnetic field, and further to reduce the aspect ratio and size of the magnetoresistive element. It is to provide a technique for doing this.

  Hereinafter, means for solving the problem will be described using the numbers and symbols used in [Best Mode for Carrying Out the Invention]. These numbers and symbols are added to clarify the correspondence between the description of [Claims] and the description of [Best Mode for Carrying Out the Invention]. However, the added numbers and symbols shall not be used for the interpretation of the technical scope of the invention described in [Claims].

  A magnetoresistive device according to the present invention comprises a free ferromagnetic layer (10) having reversible free spontaneous magnetization, a fixed ferromagnetic layer (8) having fixed fixed spontaneous magnetization, a free ferromagnetic layer (10), A magnetoresistive element (4) including a tunnel insulating layer (9) interposed between the fixed ferromagnetic layer (8) and a nonmagnetic element for electrically connecting the magnetoresistive element (4) to another element A conductor (2, 3, 11, 22) and a diffusion prevention structure (13, 14, 21) provided between the conductor (2, 3, 11, 22) and the magnetoresistive element (4). ing. The nonmagnetic conductors (2, 3, 11, and 22) that electrically connect the magnetoresistive element (4) to other elements are typically vias and wiring layers.

  The diffusion prevention structure (13, 14, 21) is formed so as to have a function of preventing at least one of the materials constituting the conductor (2, 3, 11, 22) from diffusing into the magnetoresistive element (4). Is done. Furthermore, the diffusion preventing structure (13, 14, 21) is formed so as to have a function of preventing at least one of the materials constituting the magnetoresistive element (4) from diffusing into the magnetoresistive element (4). .

  The diffusion preventing structure (13, 14, 21) diffuses from the conductor (2, 3, 11, 22) to the magnetoresistive element (4) and from the magnetoresistive element (4) to the conductor (2, 3, 11, It is important to have the function of preventing both diffusion to 22). One of the diffusion from the conductor (2, 3, 11, 22) to the magnetoresistive element (4) and the diffusion from the magnetoresistive element (4) to the conductor (2, 3, 11, 22) is It causes the other occurrence. Therefore, it is extremely preferable that the oxide layers (13, 14, 21) prevent both of these two diffusion forms from the viewpoint of improving the characteristics of the magnetoresistive element (4).

  Such a structure is effective when the conductor (2, 3, 11, 22) contains at least one element selected from the group consisting of Al, Cu, Ta, Ru, Zr, Ti, Mo, and W. is there.

The diffusion preventing structure (14) is preferably formed of one material selected from the group consisting of oxides, nitrides, and oxynitrides. These materials can be easily formed densely and have a large diffusion preventing effect. It is preferable that the diffusion prevention structure (14) is formed of a conductive nitride because the resistance of the diffusion prevention structure (14) is reduced and the SN ratio of the magnetoresistive element (4) is effectively improved. The diffusion preventing structure (14) is preferably composed of an oxide of an element that has an equal or greater ease of bonding to oxygen with respect to an element constituting a layer in contact with the upper and lower surfaces. The diffusion preventing structure (14) is preferably formed of a nitride of an element having a lower free energy for nitride formation than an element constituting a layer in contact with the upper and lower surfaces thereof. Similarly, the diffusion preventing structure (14) is preferably formed of an oxynitride of an element having lower oxide formation free energy and lower nitride formation free energy than the elements constituting the layers in contact with the upper and lower surfaces thereof. It is. Specifically, the diffusion prevention structure (14) is made of AlO _x , MgO _x , SiO _x , TiO _x , CaO _x , LiO _x , HfO _x , AlN, AlNO, SiN, SiNO , TiNO, BN , HfNO, ZrN. Preferably, it is formed of a material selected from the group consisting of:

  When the diffusion prevention structure (13, 14, 21) is formed of an oxide, the tunnel insulating layer (9) and the diffusion prevention structure (13, 14, 21) may be formed of the same material. Is preferred. Such a structure effectively reduces the cost required for forming the tunnel insulating layer (9) and the diffusion prevention structure (13, 14, 21). In this case, the diffusion prevention structure (13, 14, 21) is preferably thinner than the tunnel insulating layer (9). In addition, in order to increase the S / N ratio for detecting the direction of spontaneous magnetization of the free ferromagnetic layer (10), the resistance in the thickness direction of the oxide layers (13, 14, 21) is reduced by the tunnel insulating layer (9). It is preferable that the resistance is lower than the resistance in the thickness direction.

  A conductor (2, 3, 11, 22) electrically connected to the pinned ferromagnetic layer (8) without passing through the tunnel insulating layer (9), and a tunnel insulating layer ( 9) including the second conductor (3, 22) that is electrically connected to the free ferromagnetic layer (10) without the interposition, the diffusion prevention structure (13, 14, 21) includes the first conductor (13, 14, 21). 2, 11) and the fixed ferromagnetic layer (8), between the first diffusion prevention layer (13), the second conductor (3, 22) and the free ferromagnetic layer (10). It is preferable to include an intervening second diffusion preventing layer (14, 21). Such a structure provides interdiffusion between the pinned ferromagnetic layer (8) and the first conductor (2, 11) and between the free ferromagnetic layer (10) and the second conductor (2, 22). It effectively prevents both interdiffusion. The first diffusion prevention layer (13) and the second diffusion prevention layer (14, 21) may be formed of one material selected from the group consisting of oxide, nitride, and oxynitride. Is preferred.

  The magnetoresistive device may include a layer (7) containing Mn. A layer formed of an antiferromagnetic material such as PtMn or IrMn is a typical Mn-containing layer (7). In this case, it is preferable that the oxide layer (13) is interposed between the layer (7) containing Mn and the conductors (2, 11). Further, the magnetoresistive device may include a layer (10) containing Ni. A layer formed of a magnetically soft ferromagnet such as NiFe is a typical Ni-containing layer (7). In this case, the oxide layers (14, 21) are preferably interposed between the Ni-containing layer (10) and the conductors (3, 22).

  The conductor (2, 3, 11, 22) includes a first conductor (2, 11) that is electrically connected to the fixed ferromagnetic layer (8) without the tunnel insulating layer (9), and the oxide layer (13, 14, 21) is formed of an oxide, and a first oxide layer (13) interposed between the pinned ferromagnetic layer (8) and the first conductor (2, 11) is provided. The magnetoresistive element (4) is bonded to the pinned ferromagnetic layer (8) and further includes an antiferromagnetic layer (7) containing Mn, the antiferromagnetic layer (7) It is preferably located between the layer (8) and the first oxide layer (13). Such a structure can effectively prevent diffusion of Mn from the antiferromagnetic layer (7) to the first conductor (2, 11).

  The fixed ferromagnetic layer (8) includes a ferromagnetic layer (8b) directly joined to the tunnel insulating layer (9) and a composite inserted between the ferromagnetic layer (8b) and the antiferromagnetic layer (7). The composite magnetic layer (8a) includes a non-oxidized metal ferromagnet as a main component, is more easily coupled to oxygen than the metal ferromagnet, and is a nonmagnetic element. It is preferable that the oxide is formed by mixing as an accessory component. The composite magnetic layer (8a) having such a structure transmits exchange interaction from the antiferromagnetic layer (7) to the ferromagnetic layer (8b), while tunnel insulation of Mn contained in the antiferromagnetic layer (7). Effectively prevents diffusion into layer (9). As a result, the composite magnetic layer (8a) and the ferromagnetic layer (8b) can function as the fixed ferromagnetic layer (8), and deterioration of the magnetic tunnel junction due to Mn diffusion can be suppressed. The ferromagnetic material contained in the ferromagnetic layer (8b) and the composite magnetic layer (8a) is preferably formed of a metallic ferromagnetic alloy containing Co as a main component. Co has a high spin polarizability, is not easily oxidized, and is thermally stable and difficult to diffuse.

  The free ferromagnetic layer (10) is mainly composed of a ferromagnetic layer (10a) that is directly bonded to the tunnel insulating layer (9), and a metal ferromagnetic material that is bonded to the ferromagnetic layer (10a) and is not oxidized. As a component, it is preferable to include a composite magnetic layer (10b) which is more easily bonded to oxygen than the metal ferromagnet and is formed by mixing an oxide of a nonmagnetic element as a subcomponent. The composite magnetic layer (10b) having such a structure is relatively magnetically soft because the magnetocrystalline anisotropy is small, and a magnetically soft free ferromagnetic layer (10) containing no Ni is formed. Make it possible to realize.

  The free ferromagnetic layer (10) contains Ni, and the conductors (2, 3, 11, 22) are electrically connected to the free ferromagnetic layer (10) without going through the tunnel insulating layer (9). When two conductors (3, 22) are included, the oxide layer (13, 14, 21) is formed of an oxide, and between the free ferromagnetic layer (10) and the second conductor (3, 22). It is preferable that the second oxide layer (14, 21) interposed between the first and second oxide layers is included. Such a structure prevents the material constituting the second conductor (3, 22) from diffusing into the magnetoresistive element (4), and at the same time prevents the Ni second conductor (3 included in the free ferromagnetic layer (10)). , 22) is prevented.

  In this case, it is preferable that the second oxide layer (14) constitutes a free ferromagnetic layer (10) and is in direct contact with the Ni-containing ferromagnetic film (10, 10c) containing Ni. This structure prevents the composition shift of the Ni-containing ferromagnetic film (10, 10c) and improves the characteristics of the magnetoresistive element (4).

  The free ferromagnetic layer (10) does not contain Ni and is joined to the first ferromagnetic layer (10a) directly joined to the tunnel insulating layer (9) and the first ferromagnetic layer, and is oxidized. A composite magnetic layer (10b) comprising a non-metallic ferromagnet as a main component, being more easily coupled to oxygen than the metal ferromagnet, and being mixed with an oxide of a nonmagnetic element as a subcomponent A second ferromagnetic layer (10c) that is bonded to the composite magnetic layer (10b), contains Ni, and is magnetically softer than the composite magnetic layer (10b) and the first ferromagnetic layer (10a); It is also suitable to contain. The composite magnetic layer (10b) prevents diffusion of Ni from the second ferromagnetic layer (10c) to the tunnel insulating layer (9), while the second ferromagnetic layer (10c) to the first ferromagnetic layer (10a). Mediates exchange interactions with This makes it possible to realize a magnetoresistive device that is magnetically soft and does not cause Ni to diffuse into the tunnel insulating layer (9). The metal ferromagnetic material included in the first ferromagnetic layer (10a) and the composite magnetic layer (10b) is preferably formed of a metal ferromagnetic alloy containing Co as a main component. Co has a high spin polarizability, is not easily oxidized, and is thermally stable and difficult to diffuse.

  The free ferromagnetic layer (10) includes a first ferromagnetic layer (10d) directly bonded to the tunnel insulating layer (9) and a metal strong magnetic layer bonded to the first ferromagnetic layer (10d) and not oxidized. A first composite magnetic layer (10e) formed by mixing a magnetic material and a nonmagnetic metal oxide, and a non-oxidized metal ferromagnetic material and a nonmagnetic metal oxide are mixed. The second composite magnetic layer (10g) formed, and interposed between the first composite magnetic layer (10e) and the second composite magnetic layer (10g), and the first composite magnetic layer (10e) and the second composite magnetic layer (10g) It is preferable to include a nonmagnetic layer (10f) that antiferromagnetically couples the composite magnetic layer (10g). The nonmagnetic layer (10f) keeps the spontaneous magnetization of the second composite magnetic layer (10g) and the spontaneous magnetization of the first composite magnetic layer (10e) and the first ferromagnetic layer (10d) antiparallel, thereby The free ferromagnetic layer (10) is made magnetically softer. Such a structure allows the free ferromagnetic layer (10) to be magnetically soft without using Ni.

  In order to magnetically soften the free ferromagnetic layer (10), the free ferromagnetic layer (10) is further bonded to the second composite magnetic layer (10g), contains Ni, and It is preferable to include a second ferromagnetic layer (10h) that is magnetically softer than the first composite magnetic layer (10e), the second composite magnetic layer (10g), and the first ferromagnetic layer (10d). Ni contained in the second ferromagnetic layer (10h) makes the free ferromagnetic layer (10) magnetically softer. The problem of Ni diffusing into the tunnel insulating layer (9) can be avoided because the first composite magnetic layer (10e) and the second composite magnetic layer (10g) prevent Ni diffusion. Furthermore, the problem that Ni diffuses into the second conductor (3, 22) electrically connected to the free ferromagnetic layer (10) is that the free ferromagnetic layer (10) and the second conductor (3, 22) This can be avoided by providing a second oxide layer (14) made of an oxide between them. The first ferromagnetic layer (10d), the metal ferromagnet included in the first composite magnetic layer (10e), and the metal ferromagnet included in the second composite magnetic layer (10g) are mainly composed of Co. It is desirable to form from a metal ferromagnetic alloy as a component. Co has a high spin polarizability, is not easily oxidized, and is thermally stable and difficult to diffuse.

  When the free ferromagnetic layer (10) includes the first composite magnetic layer (10e), the nonmagnetic layer (10f), and the second composite magnetic layer (10g), the free ferromagnetic layer (10) further includes the first Between the third ferromagnetic layer (10i) interposed between the composite magnetic layer (10e) and the nonmagnetic layer (10f), and between the second composite magnetic layer (10g) and the nonmagnetic layer (10f). And the third ferromagnetic layer (10i) and the fourth ferromagnetic layer (10i) are formed of a metal ferromagnetic layer containing Co as a main component. Is preferred.

  The magnetoresistive device further includes a magnetic bias element (20) for applying a bias magnetic field to the free ferromagnetic layer (10). The magnetic bias element (20) includes a magnetic bias ferromagnetic layer (17) and a magnetic field. When the magnetic biasing anti-ferromagnetic layer (18) containing Mn is bonded to the biasing ferromagnetic layer (17), the oxide layers (13, 14, 21) are free from the magnetic bias element (20). A first oxide layer (14) interposed between the ferromagnetic layer (10) and a second oxide interposed between the magnetic bias element (20) and the second conductors (3, 22). It is preferable that a physical layer (21) is included.

  When the conductor (2, 3, 11, 22) has the second conductor (3) electrically connected to the free ferromagnetic layer (10) without passing through the tunnel insulating layer (9), the diffusion prevention structure (13, 14, 21) preferably includes a second diffusion preventing layer (14) interposed between the free ferromagnetic layer (10).

  The second diffusion prevention layer (14) is directly joined to the free ferromagnetic layer (10), and the thickness of the free ferromagnetic layer (10) is preferably 3 nm or less, and in particular free The product of the saturation magnetization and the film thickness of the ferromagnetic layer (10) is preferably 3 (T · nm) or less.

  The free ferromagnetic layer (10) preferably includes a Ni-containing ferromagnetic film containing Ni, and the second diffusion prevention layer (14) is preferably in direct contact with the Ni-containing ferromagnetic film.

  The free ferromagnetic layer (10) includes a ferromagnetic layer (31) directly bonded to the tunnel insulating layer (9), a non-magnetic element and a ferromagnetic layer (31) bonded to the ferromagnetic layer (31). And a magnetization control structure (32, 34, 35) containing a ferromagnetic material constituting the material.

  The magnetization control structure (32, 34, 35) is preferably a non-magnetic material.

  The magnetization control structure (34) is preferably formed of an oxide or nitride of a ferromagnetic material constituting the ferromagnetic layer (31).

  The nonmagnetic material is at least one element selected from Ru, Pt, Hf, Pd, Al, W, Ti, Cr, Si, Zr, Cu, Zn, Nb, V, Cr, Mg, Ta, and Mo. It is preferable that it is comprised. The nonmagnetic material is preferably segregated at the crystal grain boundaries of the ferromagnetic material.

  The free ferromagnetic layer (10) is preferably formed so that the directions of the easy axis of the stress-induced magnetic anisotropy and the shape magnetic anisotropy coincide.

  Specifically, the free ferromagnetic layer (10) has a shape that is long in the first direction, and the magnetostriction constant of the free ferromagnetic layer (10) is positive. It is preferable that a compressive stress is applied in a second direction perpendicular to the first direction. Alternatively, it is preferable that a tensile stress is applied to the free ferromagnetic layer (10) in the first direction.

  When the magnetostriction constant of the free ferromagnetic layer (10) is negative, a compressive stress is applied to the free ferromagnetic layer (10) in the first direction or is perpendicular to the first direction. It is preferable that a tensile stress is applied in the second direction.

  The stress can be controlled by the relative direction between the direction of the long axis of the free ferromagnetic layer (10) and the lower wiring (3). Specifically, when the magnetostriction constant of the free ferromagnetic layer (10) is positive and the free ferromagnetic layer (10) has a long shape in the first direction, the lower wiring (3) It extends in the direction.

  The magnetostriction constant of the free ferromagnetic layer (10) is negative, and the junction surface where the free ferromagnetic layer (10) and the tunnel insulating layer (9) are joined is long in the first direction perpendicular to the second direction. When it has a shape, the lower wiring (3) extends in the second direction.

  The stress-induced magnetic anisotropy of the free ferromagnetic layer (10) is preferably stronger than the shape magnetic anisotropy of the free ferromagnetic layer (10). The characteristics of the free ferromagnetic layer (10) are defined as the ratio of the major axis to the minor axis, in particular, the free ferromagnetic layer (10) has a major axis and a minor axis perpendicular to the major axis. This is suitable when the aspect ratio is 1.0 or more and 2.0 or less.

The present invention provides a technique for further improving the thermal stability of the magnetoresistive element.
Further, according to the present invention, it is possible to prevent mutual diffusion between a conductor (for example, via and wiring) electrically connecting the magnetoresistive element to another element and a layer constituting the magnetoresistive element. A technique for further improving the thermal stability of the element is provided.
Further, according to the present invention, the material contained in the layer constituting the magnetoresistive element, particularly Ni, Mn, diffuses into the conductor that electrically connects the magnetoresistive element to other elements, so that the resistivity of the conductor is reduced. Techniques for preventing increasing phenomena are provided.
In addition, the present invention provides a technique for preventing a phenomenon in which the characteristics of a magnetoresistive element deteriorate due to diffusion of a material contained in a conductor electrically connecting the magnetoresistive element to another element into the magnetoresistive element. Is done.
Further, according to the present invention, Mn and Ni contained in the magnetoresistive element are added to the tunnel insulating layer of the magnetoresistive element while maintaining the magnetic coupling and electrical coupling in the fixed ferromagnetic layer and the free ferromagnetic layer of the magnetoresistive element. A technique for preventing a phenomenon in which the characteristics of the magnetoresistive element deteriorate due to diffusion is provided.
In addition, the present invention provides a technique that makes it possible to reduce the reversal magnetic field by thinning the free ferromagnetic layer.
In addition, according to the present invention, there is provided a technique capable of preventing the material constituting the free ferromagnetic layer from diffusing and making the free ferromagnetic layer thin and reducing the switching magnetic field.
Further, according to the present invention, the reversal magnetic field is reduced while maintaining the squareness of the magnetoresistance curve of the free ferromagnetic layer and suppressing the variation of the reversal magnetic field, and further, the aspect ratio and size of the magnetoresistive element are reduced. Technology is provided.

One embodiment of the magnetoresistive element according to the present invention is a cross-point cell type MRAM as shown in FIG. In the first embodiment, the lower wiring 2 and the upper wiring 3 are formed on the upper surface side of the substrate 1. The lower wiring 2 and the upper wiring 3 are formed of Al ₉₀ Cu ₁₀ .

  A magnetoresistive element 4 that functions as a memory cell of the MRAM is provided between the lower wiring 2 and the upper wiring 3. The magnetoresistive element 4 includes a seed layer 5, a buffer layer 6, an antiferromagnetic layer 7, a fixed ferromagnetic layer 8, a tunnel insulating layer 9, and a free ferromagnetic layer 10. The fixed ferromagnetic layer 8, the tunnel insulating layer 9, and the free ferromagnetic layer 10 of the magnetoresistive element 4 constitute a magnetic tunnel junction.

  The fixed ferromagnetic layer 8 is formed of a metal ferromagnetic alloy having a high spin polarizability, typically CoFe. The CoFe alloy is a ferromagnetic material that is magnetically relatively hard (that is, has a large coercive force). As will be described later, the spontaneous magnetization of the pinned ferromagnetic layer 8 is pinned by exchange interaction from the antiferromagnetic layer 7.

  The free ferromagnetic layer 10 is formed of a ferromagnetic material that is magnetically relatively soft (that is, has a small coercive force). The free magnetization of the free ferromagnetic layer 10 is reversible, and the direction of the spontaneous magnetization is either parallel or antiparallel to the spontaneous magnetization of the pinned ferromagnetic layer 8. It is formed as follows. The spontaneous magnetization of the free ferromagnetic layer 10 can be reversed, and the magnetoresistive element 4 stores 1-bit data as the direction of the spontaneous magnetization of the free ferromagnetic layer 10.

  The free ferromagnetic layer 10 whose spontaneous magnetization is reversed is formed of a ferromagnetic material including Ni, typically NiFe. In general, a ferromagnetic material containing Ni is relatively magnetically soft and suitable for forming the free ferromagnetic layer 10 whose spontaneous magnetization is reversed. The free ferromagnetic layer 10 may be composed of a CoFe layer formed on the tunnel insulating layer 9 and a NiFe layer formed on the CoFe layer. The MR ratio of the magnetic tunnel junction is improved by the CoFe layer having a high spin polarizability, while the NiFe layer softens the CoFe layer and reduces the coercive force. The laminated structure formed of the CoFe layer and the NiFe layer realizes a magnetic tunnel junction in which spontaneous magnetization can be easily reversed and the MR ratio is high.

The tunnel insulating layer 9 is formed of a nonmagnetic insulator that is thin enough to allow a tunnel current to flow. The tunnel insulating layer 9 is typically formed of AlO _x , AlN _x , and MgO _x , and the thickness thereof is set according to the resistance required for the magnetoresistive element 4, typically 1.2 to 2 nm. Due to the tunnel magnetoresistance effect (TMR effect), the resistance value in the thickness direction of the tunnel insulating layer 9 is such that the spontaneous magnetization of the fixed ferromagnetic layer 8 and the spontaneous magnetization of the free ferromagnetic layer 10 are parallel or antiparallel. It depends on. The data stored in the magnetoresistive element 4 can be discriminated by the resistance value in the thickness direction of the tunnel insulating layer 9.

  The seed layer 5 and the buffer layer 6 are provided to control the orientation of the antiferromagnetic layer 7 provided thereon and to stabilize the antiferromagnetic phase of the antiferromagnetic layer 7. The seed layer 5 is typically formed of Ta and Cr, and the buffer layer 6 provided on the seed layer 5 is typically formed of NiFe and CoFe.

  The antiferromagnetic layer 7 is formed of an antiferromagnetic material containing Mn, typically PtMn and IrMn. The antiferromagnetic layer 7 causes exchange interaction with the fixed ferromagnetic layer 8 bonded immediately above the antiferromagnetic layer 7 to fix the spontaneous magnetization of the fixed ferromagnetic layer 8.

  A via 11, a lower contact layer 12, and an oxide layer 13 are provided between the magnetoresistive element 4 and the lower wiring 2. The fixed ferromagnetic layer 8 of the magnetoresistive element 4 is electrically connected to the lower wiring 2 through the via 11, the lower contact layer 12, and the oxide layer 13. The via 11 is bonded to the lower wiring 2 and extends in a direction perpendicular to the main surface of the substrate 1. The via 11 is typically formed of tungsten, copper, or molybdenum.

  The lower contact layer 12 is provided on the via 11. The lower contact layer 12 functions as a good adhesion layer for the via 11 and improves the film quality of the oxide layer 13 thereabove. Furthermore, the lower contact layer 12 maintains good electrical contact between the via 11 and the oxide layer 13. The lower contact layer 12 is typically formed of TiN, Ta, Ru, W, Zr, and Mo.

  The oxide layer 13 effectively prevents mutual diffusion between the lower wiring 2, the via 11 and the lower contact layer 12, and the magnetoresistive element 4. That is, the oxide layer 13 includes aluminum and copper constituting the lower wiring 2, tungsten, copper, and molybdenum constituting the via 11, and TiN, Ta, Ru, W, Zr, and the lower contact layer 12. Mo is effectively prevented from diffusing into the magnetoresistive element 4. Further, the oxide layer 13 effectively prevents Ni constituting the buffer layer 6 and Mn constituting the antiferromagnetic layer 7 from diffusing into the lower wiring 2. The oxide layer 13 formed of an oxide can easily have a dense structure, and can effectively prevent mutual diffusion.

  The oxide layer 13 performs both diffusion from the lower wiring 2, the via 11 and the lower contact layer 12 to the magnetoresistive element 4, and diffusion from the magnetoresistive element 4 to the lower wiring 2, the via 11 and the lower contact layer 12. Having a function to prevent is important. One of the diffusion from the lower wiring 2, the via 11 and the lower contact layer 12 to the magnetoresistive element 4 and the diffusion from the magnetoresistive element 4 to the lower wiring 2, the via 11 and the lower contact layer 12 occurs in the other Causes the occurrence. Therefore, it is extremely preferable that the oxide layer 13 prevent both of these two diffusion forms from the viewpoint of improving the characteristics of the magnetoresistive element 4.

  The oxide layer 13 is preferably formed of an oxide of Al, Mg, Si, Hf, Li, Ca, and Ti. The oxide layer 13 formed of oxides of these elements has a very dense structure, high bonding strength with oxygen, high thermal stability, and a great effect of suppressing interdiffusion.

  In particular, in order to prevent Mn diffusion, it is preferable to use a film formed of an oxide from the viewpoint of effectively suppressing Mn diffusion. Mn is easily bonded to oxygen. For this reason, when Mn diffuses in the oxide layer 13 formed of oxide, the diffused Mn is combined with oxygen and stabilized, and is fixed to the oxide layer 13. By fixing Mn to the oxide layer 13, the diffusion of Mn into the lower wiring 2 is effectively prevented.

  The oxide layer 13 is an oxidation of a material having a higher ability to bond with oxygen than the material constituting the lower surface (the surface on the substrate 1 side) and the layer in contact with the upper surface (that is, the lower contact layer 12 and the seed layer 5). It is preferable to use a product. Using an oxide of a material other than that for the oxide layer 13 causes oxygen to diffuse into the layers in contact with the lower surface and the upper surface of the oxide layer 13 to destabilize the oxide layer 13 and prevent diffusion of the oxide layer 13. It is not suitable because the effect is lost. When Ta is used for the lower contact layer 12 and Ta or Cr is used for the seed layer 5, oxides of Al, Mg, Si, Hf, Li, Ca, and Ti, which have lower free energy for oxide formation than those, are oxidized. It is preferable to use it as the physical layer 13.

  In order to improve the SN ratio in detecting the resistance of the tunnel insulating layer 9, it is preferable that the resistance in the thickness direction of the oxide layer 13 is as small as possible. Since the oxide layer 13 is electrically connected to the magnetic tunnel junction in series, the resistance in the thickness direction of the oxide layer 13 degrades the SN ratio in detecting the resistance of the tunnel insulating layer 9. Therefore, the resistance in the thickness direction of the oxide layer 13 is preferably small, and more specifically, it is preferably smaller than the resistance in the thickness direction of the tunnel insulating layer 9.

  The oxide layer 13 functioning as a diffusion preventing layer is preferably thin even when its specific resistance is small (that is, when the thickness of the oxide layer 13 can be increased). When the oxide layer 13 is grown thickly, the lattice distortion of the oxide layer 13 increases, and therefore undesirably stresses the free ferromagnetic layer 10. The stress applied to the free ferromagnetic layer 10 is not preferable because it changes the magnetic anisotropy of the free ferromagnetic layer 10 and makes it difficult to suitably control the characteristics of the free ferromagnetic layer 10. Specifically, the thickness of the oxide layer 13 is preferably 5 nm or less.

  In addition, it is preferable that the thickness of the oxide layer 13 is 1 nm or less because the resistance in the thickness direction of the oxide layer 13 can be substantially eliminated. By setting the thickness of the oxide layer 13 to 1 nm or less, the resistance of the oxide layer 13 becomes extremely small due to a tunnel phenomenon.

  The formation of the tunnel insulating layer 9 and the oxide layer 13 with the same material means that the tunnel insulating layer 9 and the oxide layer 13 can be formed with a common device and material, and the cost required for manufacturing the MRAM can be reduced. It is suitable. For example, when the tunnel insulating layer 9 and the oxide layer 13 are formed by sputtering, the tunnel insulating layer 9 and the oxide layer 13 are formed by forming the tunnel insulating layer 9 and the oxide layer 13 with the same material. It becomes possible to form using the same sputter target.

  When the tunnel insulating layer 9 and the oxide layer 13 are formed of the same material, in order to improve the SN ratio in detecting the resistance of the tunnel insulating layer 9, the thickness of the oxide layer 13 is set to It is preferable that the film thickness is smaller than 9. Thereby, the resistance in the thickness direction of the oxide layer 13 becomes smaller than the resistance in the thickness direction of the tunnel insulating layer 9, and the SN ratio in detecting the resistance of the tunnel insulating layer 9 can be improved.

  An oxide layer 14 and an upper contact layer 15 are provided between the upper wiring 3 and the magnetoresistive element 4. The oxide layer 14 is formed on the free ferromagnetic layer 10 of the magnetoresistive element 4. The upper contact layer 15 is formed on the oxide layer 14 and joined to the upper wiring 3. The free ferromagnetic layer 10 of the magnetoresistive element 4 is electrically connected to the upper wiring 3 through the oxide layer 14 and the upper contact layer 15.

  The upper contact layer 15 protects layers below the upper contact layer 15 from damage due to the element processing process, and maintains good electrical contact between the oxide layer 14 and the upper wiring 3. The upper contact layer 15 is typically formed of TiN, Ta, Ru, W, Zr, and Mo.

  The oxide layer 14 effectively prevents mutual diffusion between the upper wiring 3 and the magnetoresistive element 4. That is, the oxide layer 14 prevents diffusion of aluminum and copper constituting the upper wiring 3 into the magnetoresistive element 4 and prevents diffusion of Ni contained in the free ferromagnetic layer 10 into the upper wiring 3. By suppressing the diffusion of Ni into the upper wiring 3, the resistance of the upper wiring 3 is prevented from increasing. The oxide layer 14 formed of an oxide can easily have a dense structure, and can effectively prevent mutual diffusion. For the same reason as the oxide layer 13, the oxide layer 14 formed of an oxide is preferable in that it effectively suppresses the diffusion of Ni.

The fact that the oxide layer 14 directly connected to the free ferromagnetic layer 10 prevents the diffusion of Ni from the free ferromagnetic layer 10 to the upper wiring 3 reduces the thickness t of the free ferromagnetic layer 10, Therefore, it is important also in enabling reduction of M _s × t of the free ferromagnetic layer 10. The outflow of Ni from the free ferromagnetic layer 10 causes the composition of the free ferromagnetic layer 10 to fluctuate greatly, particularly when the film thickness t of the free ferromagnetic layer 10 is thin, and makes the characteristics of the free ferromagnetic layer 10 unstable. . The insertion of the oxide layer 14 prevents Ni from flowing out of the free ferromagnetic layer 10 and makes it possible to adjust the composition of the free ferromagnetic layer 10 to a desired value. This is particularly effective when the film thickness t of the free ferromagnetic layer 10 is thin.

  Like the oxide layer 13, it is important that the oxide layer 14 has a function of preventing both diffusion from the upper wiring 3 to the free ferromagnetic layer 10 and diffusion from the free ferromagnetic layer 10 to the upper wiring 3. is there. It is extremely preferable that the oxide layer 13 prevent both of these diffusions from the viewpoint of improving the characteristics of the magnetoresistive element 4.

  The physical properties required for the oxide layer 14 are the same as those of the oxide layer 13, and materials and structures suitable for the oxide layer 13 are also suitable for the oxide layer 14. First, in order to refine the structure of the oxide layer 14, maintain thermal stability up to a high temperature, and increase the interdiffusion suppression effect, the oxide layer 14 includes Al, Mg, Si, It is preferable to form with oxides of Hf, Li, Ca, and Ti. Further, in order to improve the S / N ratio for detecting the resistance of the tunnel insulating layer 9, the resistance in the thickness direction of the oxide layer 14 is preferably smaller than the resistance in the thickness direction of the tunnel insulating layer 9. Furthermore, it is preferable that the thickness of the oxide layer 14 is thin, in particular, 5 nm or less, from the viewpoint of suppressing the influence of the stress on the free ferromagnetic layer 10. Furthermore, forming the tunnel insulating layer 9 and the oxide layer 14 with the same material enables the tunnel insulating layer 9 and the oxide layer 14 to be formed with a common device and material, thereby reducing the cost required for manufacturing the MRAM. It is preferable in that it can be performed.

  When the free ferromagnetic layer 10 is formed of a ferromagnetic layer containing Ni (typically a NiFe layer), or a laminated structure of a ferromagnetic layer containing Ni and another ferromagnetic layer. When it has (typically when it has a laminated structure of a CoFe layer and a NiFe layer), the oxide layer 14 may be formed directly on the ferromagnetic layer containing Ni. Is preferred. The characteristics of the ferromagnetic layer containing Ni are likely to deteriorate if the Ni composition deviates from the optimum composition. For example, the magnitude of saturation magnetization is greatly affected by the composition of Ni. The fact that the oxide layer 14 is in direct contact with the ferromagnetic layer containing Ni eliminates the diffusion path of Ni above the ferromagnetic layer, resulting in a composition shift of the ferromagnetic layer containing Ni. Effectively prevent. The ferromagnetic layer containing Ni is a layer in contact with the upper surface (that is, the upper contact layer 15 and the upper wiring 3 as compared with the layer in contact with the lower surface (that is, the tunnel insulating layer 9 or the other ferromagnetic layer described above)). It is known from experiments by the inventors and the like that the formation of the oxide layer 14 directly on the upper surface of the ferromagnetic layer containing Ni is particularly effective for preventing diffusion. is there.

  As described above, in the present embodiment, the oxide layer 13 is provided between the lower wiring 2 and the magnetoresistive element 4, and mutual diffusion between the lower wiring 2 and the magnetoresistive element 4 is prevented. It is effectively prevented. Furthermore, an oxide layer 14 is provided between the upper wiring 3 and the magnetoresistive element 4, and mutual diffusion between the upper wiring 3 and the magnetoresistive element 4 is effectively prevented. Even when Cu is used for the lower wiring 2 and the upper wiring 3, the use of the oxide layers 13 and 14 is effective in preventing mutual diffusion.

Furthermore, in this embodiment, since the oxide layer 14 is formed on the free ferromagnetic layer 10, the diffusion of the material constituting the free ferromagnetic layer 10, particularly Ni, is effectively prevented. This makes it possible to stabilize the product M _s × t of the magnetization M _s and the film thickness t of the free ferromagnetic layer 10 to a small value even when the film thickness t of the free ferromagnetic layer 10 is thin. As described above, reducing the product M _s × t is important for reducing and stabilizing the switching magnetic field of the free ferromagnetic layer 10. The structure of the present embodiment is particularly effective when the film thickness t of the free ferromagnetic layer 10 is 3 nm or less and when the product M _s × t is 3 (T · nm) or less.

  In the present embodiment, a nitride layer formed of nitride may be used instead of the oxide layer 13 and the oxide layer 14. The nitride layer or the oxynitride layer can easily have a dense structure, and can effectively prevent mutual diffusion. Like the oxide layer 13 and the oxide layer 14, the nitride layer preferably has a thickness of 5 nm or less. A thick nitride layer is undesirable because it can cause degradation of magnetic properties due to stress induced magnetic anisotropy.

  It is preferable to form the nitride layer with a nitride which is a conductor because the resistance in the thickness direction of the nitride layer is reduced and the SN ratio in the detection of the resistance of the tunnel insulating layer 9 is improved. . The resistance in the thickness direction of the nitride layer is preferably small, and more specifically, it is preferably smaller than the resistance in the thickness direction of the tunnel insulating layer 9.

As the nitride layer, it is preferable to use a nitride of a material having a lower free energy for nitride formation than the material constituting the lower surface (the surface on the substrate 1 side) and the layer in contact with the upper surface. Use of a non-nitride material for the nitride layer may cause the nitride layer to become unstable by diffusing nitrogen into the lower layer and the upper layer of the nitride layer, thereby losing the diffusion preventing effect of the nitride layer. Therefore, it is not suitable. As the nitride constituting the nitride layer, AlN, SiN , BN , and ZrN can be suitably used.

  In this embodiment, an oxynitride layer formed of oxynitride may be used instead of the oxide layer 13 and the oxide layer 14. The oxynitride layer can easily have a dense structure, and can effectively prevent mutual diffusion. Similar to the nitride layer, the oxynitride layer preferably has a thickness of 5 nm or less. A thick oxynitride layer is undesirable because it can cause degradation of magnetic properties due to magnetic anisotropy induced by stress.

  As the oxynitride layer, an oxynitride of a material having lower oxide formation free energy and lower nitride formation free energy than the material constituting the lower surface (the surface on the substrate 1 side) and the layer in contact with the upper surface is used. Is preferred. Using other oxynitride materials for the oxynitride layer destabilizes the oxynitride layer by diffusing oxygen and / or nitrogen into the layer in contact with the lower surface and the upper surface of the oxynitride layer. This is not preferable because the effect of preventing diffusion of the physical layer is lost. As the nitride constituting the oxynitride layer, AlN, SiN, TiN, BN, TaN, and ZrN can be suitably used.

As described above, the formation of the oxide layer 14 (or nitride film or oxynitride film) on the free ferromagnetic layer 10 enables the film thickness t of the free ferromagnetic layer 10 to be reduced. , It is possible to reduce the product M _s × t of the magnetization M _s and the film thickness t of the free ferromagnetic layer 10. However, in order to stably reduce the product M _s × t of the magnetization M _s and the film thickness t of the free ferromagnetic layer 10, there is a limit to simply reducing the film thickness t of the free ferromagnetic layer 10. . This is because, as is understood from the thin film formation process, when the film thickness t of the free ferromagnetic layer 10 is made extremely thin, the free ferromagnetic layer 10 is formed only in an island shape and becomes discontinuous. .

Such a problem can be solved by altering a part of the sufficiently thick free ferromagnetic layer 10 to thereby reduce or eliminate the magnetization of the part. This method makes it possible to reduce the effective film thickness and magnetization of the free ferromagnetic layer 10 while reducing the product M _s × t, while forming the free ferromagnetic layer 10 in a continuous structure. . Since the ferromagnetism in the portion of the free ferromagnetic layer 10 in contact with the tunnel insulating layer 9 is not affected, the MR ratio is not reduced. The free ferromagnetic layer 10 is preferably altered so that the altered portion becomes a non-magnetic material. Since the altered portion of the free ferromagnetic layer 10 becomes a non-magnetic material, the effective film thickness t of the free ferromagnetic layer 10 is reduced, and the product M _s × t can be more effectively reduced. Become.

Specifically, the formation of the free ferromagnetic layer 10 having a small product M _s × t and a continuous structure can be realized by the following method: In the first method, as shown in FIG. As shown, the free ferromagnetic layer 10 is composed of a ferromagnetic layer 31 formed of a ferromagnetic material and a diffusion layer 32 formed of a nonmagnetic metal. The ferromagnetic layer 31 is formed on the tunnel insulating layer 9, the diffusion layer 32 is formed on the ferromagnetic layer 31, and the oxide layer 14 is formed on the diffusion layer 32. The ferromagnetic layer 31 is typically made of NiFe. The nonmagnetic metal film can typically be formed of Ru, Pt, Hf, Pd, Al, W, Ti, Cr, Si, Zr, Cu, Zn, V, Cr, and Mo. When the heat treatment is performed, diffusion occurs between the ferromagnetic layer 31 and the diffusion layer 32, a part of the ferromagnetic layer 31 is altered, and the magnetization _Ms is lowered. Thus, the free ferromagnetic layer 10 having a small product M _s × t and a continuous structure is obtained. In the free ferromagnetic layer 10 having such a structure, the degree of magnetization reduction of the free ferromagnetic layer 10 can be easily adjusted by the film thickness of the diffusion layer 32. On the other hand, since the oxide layer 14 is formed, the material constituting the free ferromagnetic layer 10 is not taken away by the upper contact layer 15, so that the product M _s × t of the free ferromagnetic layer 10 is stabilized. be able to.

  The diffusion layer 32 is not necessarily required to form a continuous “layer”. The diffusion layer 32 can be formed so that the film thickness is extremely thin and the diffusion layer 32 becomes an island-shaped structure.

The diffusion layer 32 may be provided inside the free ferromagnetic layer 10 or at any position in contact with the free ferromagnetic layer 10 as long as it does not directly contact the tunnel insulating layer 9. For example, as shown in FIG. 28, the free ferromagnetic layer 10 can be composed of ferromagnetic layers 31 and 33 and a diffusion layer 32 interposed therebetween. It is not preferable that the diffusion layer 32 is in direct contact with the tunnel insulating layer 9 because the MR ratio is reduced. By the heat treatment, mutual diffusion occurs between the diffusion layer 32 and the ferromagnetic layers 31 and 33, and the free ferromagnetic layer 10 having a small product M _s × t and a continuous structure is obtained. Again, it should be noted that the diffusion layer 32 need not necessarily be a continuous “layer”.

In the second method, as shown in FIG. 29, after the ferromagnetic layer 31 is formed on the tunnel insulating layer 9, a part of the upper surface thereof is nitrided or oxidized to form the altered layer 34. Is done. The remaining portion of the ferromagnetic layer 31 and the altered layer 34 constitute the free ferromagnetic layer 10. Nitriding and oxidation of a portion of the ferromagnetic layer 31 can be achieved by exposing the upper surface of the ferromagnetic layer 31 to nitrogen plasma and oxygen plasma, respectively. When a part of the ferromagnetic layer 31 is nitrided and oxidized, the magnetization of the nitrided or oxidized part is lost or reduced, the product M _s × t is small, and the free strength having a continuous structure is obtained. The magnetic layer 10 can be formed. Instead of being nitrided or oxidized, a portion of the ferromagnetic layer 31 can be borated, chlorinated, or carbonized.

  When the altered layer 34 is formed of an oxide, the altered layer 34 and the oxide layer 14 can be simultaneously formed by the following method. After the ferromagnetic layer 31 is formed on the tunnel insulating layer 9, a metal film for forming the oxide layer 14 is formed thereon. The upper surface of the metal film is exposed to oxygen plasma. Even after the metal film is completely oxidized and the oxide layer 14 is formed, the exposure to the oxygen plasma is continued, whereby a part of the ferromagnetic layer 31 is oxidized. By controlling the exposure time to the oxygen plasma, it is possible to control the film thickness of the oxidized portion of the ferromagnetic layer 31. This is equivalent to controlling the effective film thickness of the free ferromagnetic layer 10.

  When the altered layer 34 is formed of nitride, the altered layer 34 by nitriding a part of the ferromagnetic layer 31 and the oxide layer 14 can be simultaneously formed by the following method. It is. After the ferromagnetic layer 31 is formed on the tunnel insulating layer 9, a metal film for forming the oxide layer 14 is formed thereon. The upper surface of the metal film is exposed to nitrogen plasma. Even after the metal film is completely nitrided, the exposure to the nitrogen plasma is continued, whereby a part of the ferromagnetic layer 31 is nitrided. Thereafter, the nitrided metal film is oxidized to form an oxide layer 14. The nitrided portion of the free ferromagnetic layer 10 is not oxidized due to the difference in binding force to oxygen.

The free ferromagnetic layer 10 having a small product M _s × t and a continuous structure can also be obtained by the structure shown in FIG. 30: a ferromagnetic layer 31 made of a ferromagnetic material on the tunnel insulating layer 9. Is formed. On the ferromagnetic layer 31, a doped ferromagnetic layer 35 made of the same ferromagnetic material as the ferromagnetic layer 31 and doped with a nonmagnetic metal is formed. The formed ferromagnetic layer 31 and the doped ferromagnetic layer 35 function as the free ferromagnetic layer 10. In the doped ferromagnetic layer 35, the magnetization of the ferromagnetic material 35 is reduced by doping the doped ferromagnetic layer 35 with the crystallized segregated nonmagnetic metal of the ferromagnetic material, and the product M _s × The free ferromagnetic layer 10 having a small t and a continuous structure can be formed.

As will be described in detail below, insertion of an oxide layer 14 (or nitride film or oxynitride film) prevents interdiffusion between the upper wiring 3 and the free ferromagnetic layer 10 to thereby prevent the free ferromagnetic layer 10. The technique of controlling the composition of the magnetic material to a desired value is useful for controlling the stress-induced magnetic anisotropy of the free ferromagnetic layer 10. The magnitude of the anisotropic magnetic field Hs due to the stress-induced magnetic anisotropy of the free ferromagnetic layer 10 is expressed by the following formula (3):
H _s = 3 (λ · σ) / M _s , (3)
It is represented by Here, λ is a magnetostriction constant of the free ferromagnetic layer 10, and σ is a stress applied to the free ferromagnetic layer 10. The magnetostriction constant λ depends on the composition of the free ferromagnetic layer 10. On the other hand, the oxide layer 14 effectively reduces the variation in the composition of the free ferromagnetic layers 10 of the different magnetoresistive elements 4 and the variation in the composition within one free ferromagnetic layer 10. This means that the insertion of the oxide layer 14 makes it possible to control the magnetostriction constant of the free ferromagnetic layer 10 and thus to control the stress-induced magnetic anisotropy.

31 and 32 show an MRAM structure that realizes control of stress-induced magnetic anisotropy. As shown in FIG. 31, on the substrate 1, the lower electrode 2 is formed of a metal such as Al, Cu, or AlCu. As shown in FIG. 32, the lower electrode 2 extends in the y-axis direction, and the upper wiring 3 extends in the x-axis direction. The free ferromagnetic layer 10 has a minor axis in the x-axis direction and a major axis in the y-axis direction. Such a shape gives the free ferromagnetic layer 10 shape magnetic anisotropy in the y-axis direction. As shown in FIG. 31, the lower contact layer 12, the oxide layer 13, the seed layer 5, the buffer layer 6, and the antiferromagnetic layer 7 are sequentially formed above the lower electrode 2. The lower contact layer 12 is connected to the lower electrode 2 by a via 11. On the antiferromagnetic layer 7, a fixed ferromagnetic layer 8, a tunnel insulating layer 9, and a free ferromagnetic layer 10 are sequentially formed. An oxide layer 14 and an upper contact layer 15 are sequentially formed on the free ferromagnetic layer 10, and the upper contact layer 15 is connected to the upper wiring 3 through a via 22. The composition of the free ferromagnetic layer 10 is selected so that the magnetostriction constant λ of the free ferromagnetic layer 10 is positive. When the free ferromagnetic layer 10 is formed of Ni _x Fe _1-x , the magnetostriction constant λ can be made positive by controlling x to be less than 0.82.

  As shown in FIG. 33, the above structure has the direction of the axis of easy magnetization (anisotropy constant: K2) of the stress-induced magnetic anisotropy and the easy magnetization of the shape magnetic anisotropy. The direction of the axial direction (anisotropy constant: K3) is made coincident, and thereby the characteristics of the free ferromagnetic layer 10 can be stabilized. The lower wiring 2 extending in the y-axis direction applies tensile stress in the x-axis direction (that is, the long axis direction of the free ferromagnetic layer 10) and y-axis direction (that is, the short axis direction of the free ferromagnetic layer 10). Induces compressive stress. According to the inventors' investigation, it should be noted that the influence of the stress generated by the upper wiring 3 on the free ferromagnetic layer 10 is small. Since the magnetostriction constant λ of the free ferromagnetic layer 10 is positive, the compressive stress in the x-axis direction and the tensile stress in the y-axis direction generated by the lower wiring 2 cause stress-induced magnetic anisotropy in the y-axis direction. , Make the direction of stress-induced magnetic anisotropy and shape magnetic anisotropy coincide. The fact that the directions of the easy axes of stress-induced magnetic anisotropy and shape magnetic anisotropy coincide with each other gives the free ferromagnetic layer 10 a large uniaxiality, and the free ferromagnetic layer 10 has a single domain structure. Makes it possible to This effectively stabilizes the characteristics of the free ferromagnetic layer 10. Specifically, the coincidence of the stress-induced magnetic anisotropy and the shape magnetic anisotropy with the direction of the easy axis of magnetization improves the squareness ratio of the magnetic field-magnetization curve of the free ferromagnetic layer 10 and further varies the coercivity. Suppress. The structure of the MRAM in which the directions of the stress-induced magnetic anisotropy and the shape magnetic anisotropy do not coincide with each other changes the easy axis direction of magnetization of the uniaxial anisotropy with respect to the write wiring. Undesirably destabilize.

  In order to further stabilize the characteristics of the free ferromagnetic layer 10, the direction of the easy axis of the magnetocrystalline anisotropy of the free ferromagnetic layer 10 is stress-induced magnetic anisotropy and shape magnetic anisotropy. It is preferable that it is formed so as to coincide with the direction of the easy axis of magnetization. The direction of the easy axis of magnetization of the magnetocrystalline anisotropy K1 matches the direction of the easy axis of magnetization of the stress-induced magnetic anisotropy K2 and the shape magnetic anisotropy K3. , Thereby stabilizing the characteristics of the free ferromagnetic layer 10.

FIG. 34 shows another MRAM structure that realizes control of stress-induced magnetic anisotropy. The lower wiring 2 extends in the y-axis direction, and the upper wiring 3 extends in the x-axis direction. The magnetoresistive element 4 is formed such that the long axis of the free ferromagnetic layer 10 is in the x-axis direction and the short axis is in the y-axis direction. In the structure shown in FIG. 34, the major axis and minor axis directions of the free ferromagnetic layer 10 are 90 ° different from the major axis and minor axis directions of the free ferromagnetic layer 10 shown in FIG. Please note that. Such a shape gives the free ferromagnetic layer 10 shape magnetic anisotropy in the x-axis direction. The composition of the free ferromagnetic layer 10 is selected so that the magnetostriction constant λ of the free ferromagnetic layer 10 is negative. When the free ferromagnetic layer 10 is made of Ni _x Fe _1-x , the magnetostriction constant λ can be made negative by controlling x to a value larger than 0.82.

  The structure of FIG. 34 is the same as the structure shown in FIGS. 31 and 32, the direction of the easy axis of stress-induced magnetic anisotropy (K2) and the direction of the easy axis of shape magnetic anisotropy (K3). Thus, the characteristics of the free ferromagnetic layer 10 can be stabilized. As described above, the lower wiring 2 extending in the y-axis direction applies tensile stress in the x-axis direction (that is, the short axis direction of the free ferromagnetic layer 10) and y-axis direction (that is, the free ferromagnetic layer 10). Compressive stress is induced in the major axis direction). Since the magnetostriction constant λ of the free ferromagnetic layer 10 is negative, the compressive stress in the x-axis direction and the tensile stress in the y-axis direction generated by the lower wiring 2 cause stress-induced magnetic anisotropy in the x-axis direction. The directions of easy axes of the stress-induced magnetic anisotropy and the shape magnetic anisotropy of the free ferromagnetic layer 10 are matched.

  When the MRAM is formed so that the directions of the easy axes of stress-induced magnetic anisotropy and shape magnetic anisotropy coincide with each other, as shown in FIG. It is preferable that the anisotropy is formed to be larger than the shape magnetic anisotropy. Realization of such characteristics makes it possible to reduce the area of the magnetoresistive element 4 by making the aspect ratio of the free ferromagnetic layer 10 (that is, the ratio of the long axis to the short axis) close to 1. Generally, bringing the aspect ratio of the free ferromagnetic layer 10 close to 1 can weaken the uniaxial magnetic anisotropy of the free ferromagnetic layer 10 and cause the free ferromagnetic layer 10 to form a circulating magnetic domain. The formation of the reflux magnetic domain degrades the squareness ratio of the magnetic field-magnetization curve of the free ferromagnetic layer 10 and increases the coercivity variation. However, by forming the free ferromagnetic layer 10 so that the stress-induced magnetic anisotropy is larger than the shape magnetic anisotropy, the uniaxiality of the free ferromagnetic layer 10 is increased and the uniaxial due to the small aspect ratio. The decrease in sex is compensated. Therefore, the stress-induced magnetic anisotropy larger than the shape magnetic anisotropy enables the aspect ratio of the free ferromagnetic layer 10 to approach 1. Typically, a technique for making the stress-induced magnetic anisotropy larger than the shape magnetic anisotropy is a free ferromagnetic layer 10 having an aspect ratio of 1.0 to 2.0, particularly an aspect ratio of 1.25. It is suitable to apply to the free ferromagnetic layer 10 of 2.0 or less. A stress-induced magnetic anisotropy larger than the shape magnetic anisotropy can be realized by controlling the magnetostriction constant λ to a desired value through appropriate selection of the composition of the free ferromagnetic layer 10. The insertion of the oxide layer 14 (or nitride, oxynitride) is important in that it makes it possible to appropriately control the composition of the free ferromagnetic layer 10.

  The technique for making the stress-induced magnetic anisotropy larger than the shape magnetic anisotropy is also preferable in that the magnetoresistive element 4 that is resistant to the shape variation inevitably generated in the manufacturing process of the MRAM can be produced. Conventional magnetoresistive elements based on shape magnetic anisotropy are susceptible to shape variations caused by device processing (exposure and etching), and have large variations in coercive force. The technology to make the stress-induced magnetic anisotropy larger than the shape magnetic anisotropy reduces the influence of the inevitable shape variation and effectively suppresses the variation in coercive force.

  From the viewpoint of preventing interdiffusion, a sufficiently thick lower contact layer 12 ′ is interposed between the magnetoresistive element 4 and the via 11 in place of the oxide layer 13 as shown in FIG. Can be done. In this case, the seed layer 5 is not required. Similarly, as shown in FIG. 3, a sufficiently thick upper contact layer 15 ′ may be interposed between the free ferromagnetic layer 10 and the upper wiring 3 instead of the oxide layer 14. Is possible. The lower contact layer 12 'and the upper contact layer 15' are typically formed of TiN, Ta, Ru, W, Zr, and Mo. Since the lower contact layer 12 ′ and the upper contact layer 15 ′ are sufficiently thick, Al and Cu constituting the lower wiring 2 and the upper wiring 3 are diffused into the magnetoresistive element 4. It is possible to prevent Mn contained in the magnetic layer 7 and Ni contained in the buffer layer 6 and the free ferromagnetic layer 10 from diffusing into the lower wiring 2 and the upper wiring 3.

  However, it is preferred that both oxide layer 13 and oxide layer 14 be used, as shown in FIG. The oxide layer 13 and the oxide layer 14 formed of an oxide having a large effect of preventing mutual diffusion can be made extremely thin. The thin oxide layer 13 and oxide layer 14 make it possible to dispose the lower wiring 2 and the upper wiring 3 close to the free ferromagnetic layer 10. The fact that the lower wiring 2 and the upper wiring 3 are arranged close to the free ferromagnetic layer 10 means that the magnitude of the current necessary for reversing the spontaneous magnetization of the free ferromagnetic layer 10 (ie, for the write operation). This is preferable in order to suppress the required current magnitude).

  Further, the structure having the thick lower contact layer 12 ′ in FIG. 2 prevents the lower wiring 2 and the mutual diffusion between the via 11 and the magnetoresistive element 4, while the lower contact layer 12 ′ and the magnetoresistive element 4 are prevented. Mutual diffusion between the two cannot be prevented. Similarly, the structure having the thick upper contact layer 15 ′ of FIG. 3 prevents mutual diffusion between the upper wiring 2 and the magnetoresistive element 4, while preventing the interdiffusion between the upper contact layer 15 ′ and the magnetoresistive element 4. Mutual diffusion cannot be prevented. For this reason, the use of the oxide layer 13 is preferable to the use of the thick lower contact layer 12 ′, and the use of the oxide layer 14 is preferable to the use of the thick upper contact layer 15 ′.

  In order to dispose the lower wiring 2 closer to the free ferromagnetic layer 10, as shown in FIG. 4, the oxide layer 13, the seed layer 5, the buffer layer 6, the antiferromagnetic layer 7, However, it is preferable that the upper surface of the lower wiring 2 is formed to be substantially completely covered. Such a structure makes it possible to make the seed layer 5, the buffer layer 6, and the antiferromagnetic layer 7 a part of the wiring through which a write current for reversing the spontaneous magnetization of the free ferromagnetic layer 10 flows. It is possible to bring the wiring through which the current flows closer to the free ferromagnetic layer 10.

  As described in the prior art, it is not preferable that Mn contained in the antiferromagnetic layer 7 diffuses into the tunnel insulating layer 9 in terms of degrading the MR ratio of the magnetic tunnel junction. In order to prevent Mn contained in the antiferromagnetic layer 7 from diffusing into the tunnel insulating layer 9, the fixed ferromagnetic layer 8 is preferably formed of a composite magnetic layer 8a and a metal ferromagnetic layer 8b. It is. The composite magnetic layer 8 a is formed on the antiferromagnetic layer 7 and prevents Mn of the antiferromagnetic layer 7 from diffusing into the tunnel insulating layer 9 as will be described later. For the metal ferromagnetic layer 8b, it is desirable to use a metal ferromagnetic alloy whose main component is Co, which has a high spin polarizability and is thermally stable and difficult to diffuse. By being formed of a metal ferromagnetic alloy containing Co as a main component, the metal ferromagnetic layer 8b becomes magnetically hard. “Co as a main component” means that the constituent having the highest atomic percentage among the constituents of the metal ferromagnetic alloy is Co.

The composite magnetic layer 8a is composed of a non-oxidized metal ferromagnet as a main component, which is more easily bonded to oxygen than the metal ferromagnet, and is mixed with an oxide of a nonmagnetic element as a subcomponent. It is formed of a thin film. Such a composite magnetic layer 8a has a structure capable of preventing Mn diffusion while maintaining metallic conductivity and ferromagnetism. Typically, CoFe is exemplified as the metal ferromagnet composing the composite magnetic layer 8a, and TaO _x , HfO _x , NbO _x , ZrO _x , CeO _x , AlO _x , MgO _x , SiO are exemplified as oxides. _x and TiO _x are exemplified. These nonmagnetic elements have a lower oxide formation free energy than the ferromagnetic elements Fe, Co, and Ni, and are easily oxidized. As the ferromagnetic material used for the composite magnetic layer 8a, it is desirable to use Co and a metal ferromagnetic alloy containing Co as a main component. Co and metallic ferromagnetic alloys containing Co as a main component have high spin polarizability, are not easily oxidized, and are thermally stable and difficult to diffuse.

  It is important that the composite magnetic layer 8a has a metal ferromagnet which is not oxidized as a main component in order for the composite magnetic layer 8a to exhibit conductivity and ferromagnetism. The composite magnetic layer 8a having metallic conductivity improves the SN ratio of the read operation. The fact that the composite magnetic layer 8a has ferromagnetism causes the exchange interaction of the antiferromagnetic layer 7 to reach the metal ferromagnetic layer 8b, so that both the composite magnetic layer 8a and the metal ferromagnetic layer 8b are fixed ferromagnetic layers. Play a role to function. In order to prevent oxidation of the metal ferromagnet constituting the composite magnetic layer 8a, an oxide of a nonmagnetic element that is more easily oxidized than the metal ferromagnet is used as the oxide constituting the composite magnetic layer 8a. The

The composite magnetic layer 8a has one of the structures shown in FIGS. 6A and 6B according to the atomic radius of the element constituting the metal ferromagnet and the atomic radius of the nonmagnetic element constituting the oxide. Take. When the atomic radius of the nonmagnetic element constituting the oxide is larger than the atomic radius of the element constituting the metal ferromagnet, the composite magnetic layer 8a has a metal structure as shown in FIG. It consists of a columnar single crystal 31 of a ferromagnetic material and an amorphous phase 32 of a mixture of a metal ferromagnetic material and an oxide of a nonmagnetic element. The reason why the composite magnetic layer 8a has such a structure is considered to be that crystallization of the metal ferromagnet is inhibited by a nonmagnetic element having a large atomic radius. When the CoFe is used as the metal ferromagnetic substance of the composite magnetic layer 8a are, _TaO x as an oxide contained in the composite magnetic layer _8a, HfO _x, NbO _x, if the ZrO _x, and CeO _x is used A structure as shown in FIG.

When the atomic radius of the nonmagnetic element constituting the oxide is smaller than the atomic radius of the element constituting the metal ferromagnet, the composite magnetic layer 8a is made of metal as shown in FIG. It consists of a ferromagnetic granular single crystal 33 and an amorphous oxide 34 formed by depositing an oxide at the grain boundary of the granular single crystal 33. A material having such a structure is sometimes called a granular alloy. However, the granular single crystal 33 is not completely isolated, and a part of the granular single crystal 33 is in direct contact with the adjacent granular single crystal 33 or pinholes existing in the nonmagnetic oxide 34. It is in contact with the adjacent granular single crystal 33. In such a structure, since the granular single crystals 33 are magnetically coupled to each other, the composite magnetic layer 8a exhibits soft ferromagnetism and metallic conductivity. When CoFe is used as the metal ferromagnet constituting the composite magnetic layer 8a, the composite magnetic layer 8a is formed when AlO _x , MgO _x , SiO _x , and TiO _x are used as oxides as shown in FIG. ).

  6A and 6B, the composite magnetic layer 8a has a structure that prevents diffusion due to the denseness of the nonmagnetic oxide contained therein. Furthermore, the composite magnetic layer 8a including an oxide has a function of trapping Mn that is easily bonded to oxygen. That is, when Mn diffuses into the composite magnetic layer 8a including the oxide, the diffused Mn is stabilized by being combined with oxygen and fixed to the composite magnetic layer 8a. Furthermore, the composite magnetic layer 8a has almost no crystal grain boundaries as those of a normal metal ferromagnetic layer and has a high diffusion preventing property because the high-speed diffusion path is reduced. By these actions, the composite magnetic layer 8a has a property that can effectively prevent the diffusion of Mn into the tunnel insulating layer 9 without breaking the magnetic and electrical coupling in the pinned ferromagnetic layer. Such characteristics cannot be obtained by a conventional oxide diffusion prevention layer.

  The composite magnetic layer 8a can be formed by reactive sputtering using a sputtering gas containing oxygen gas. Typically, a mixed gas of oxygen gas and argon gas is used as the sputtering gas. As the sputtering target, a target formed of an alloy of a metal ferromagnet and a nonmagnetic element that is more easily oxidized than the metal ferromagnet is typically used. When this alloy target is sputtered using a sputtering gas containing oxygen gas, oxygen is bonded to the nonmagnetic metal in preference to the metal ferromagnet. By appropriately adjusting the amount of oxygen contained in the sputtering gas, only the nonmagnetic metal can be oxidized, and the composite magnetic layer 8a can be formed without oxidizing the metal ferromagnet.

FIG. 7 shows a thin film formed by sputtering a (Co ₉₀ Fe ₁₀ ) ₈₅ Ta ₁₅ alloy target made of CoFe as a ferromagnetic material and Ta as a nonmagnetic element with a sputtering gas containing oxygen gas. FIG. 8 is a graph showing the saturation magnetization of the thin film. The ratio [O ₂ ] / [Ar] is plotted on the horizontal axis of these graphs. Here, [O ₂ ] is the flow rate (sccm) of oxygen gas introduced into the chamber where sputtering is performed, and [Ar] is the flow rate (sccm) of argon gas. The formed thin film, the structure shown in FIG. 6 (a), i.e., the composition _formula: oxide amorphous layer having a composition represented by _{(CoFe) z Ta 1-z} O x is the majority one It has a structure in which CoFe columnar crystal grains are formed in the part. As shown in FIGS. 7 and 8, when [O ₂ ] / [Ar] is small, the thin film exhibits metallic conductivity and has a large saturation magnetization. When [O ₂ ] / [Ar] exceeds 0.2, the resistivity of the thin film increases rapidly, and the saturation magnetization decreases rapidly.

These graphs show that [O ₂ ] / [Ar] needs to be smaller than 0.2 in order for CoFe contained in the thin film to exist in a metallic state. This reasoning is supported by analysis using XPS (X-ray Photoelectron Spectroscopy). FIG. 9 shows a Co _2p spectrum obtained by analyzing the thin film prepared with [O ₂ ] / [Ar] of 0.13 and 0.54 by XPS. In the Co _2p spectrum of FIG. 9, when [O ₂ ] / [Ar] is 0.13, 70% or more of Co contained in the thin film is in a metal state, and [O ₂ ] / [Ar ] Of 0.54 indicates that Co contained in the thin film is oxidized.

When CoFe is used as the metal ferromagnet and any of TaO _x , HfO _x , NbO _x , ZrO _x , AlO _x , MgO _x , and SiO _x is used as the nonmagnetic metal oxide, [ If O ₂ ] / [Ar] is smaller than 0.2, the composite magnetic film 8a can be formed with CoFe in a metallic state.

  The composite thin film formed of the above-mentioned non-oxidized metal ferromagnet as a main component, which is easier to combine with oxygen than the metal ferromagnet and is formed by using a non-magnetic element oxide as a subcomponent, Since the metal ferromagnetic layer loses its crystal structure, the ferromagnetic crystal grains become smaller, the ferromagnetic crystal grains become smaller, or the magnetocrystalline anisotropy is reduced, so that it is relatively magnetically soft. This property can be used to eliminate Ni from the free ferromagnetic layer 10. When Ni diffuses into the tunnel insulating layer 9, the MR ratio of the magnetoresistive element 4 decreases. Further, when Ni diffuses into the upper wiring 3, the resistance of the upper wiring 3 increases. From the viewpoint of Ni diffusion, Ni is preferably excluded from the free ferromagnetic layer 10.

  FIG. 10 shows a structure in which Ni is excluded from the free ferromagnetic layer 10 using such a composite thin film. The free ferromagnetic layer 10 is composed of a metal ferromagnetic layer 10a made of a material having a high spin polarizability, thermally stable and difficult to diffuse, and a composite magnetic layer 10b made of the above-described composite thin film. Is formed. The metal ferromagnetic layer 10a is formed on the tunnel insulating layer 9, and the composite magnetic layer 10b is formed on the metal ferromagnetic layer 10a. The metal ferromagnetic layer 10a is preferably a ferromagnetic alloy containing Co as a main component, and is typically formed of CoFe. The composite magnetic layer 10b is formed of a composite thin film in which a ferromagnetic material not containing Ni (typically CoFe) and a nonmagnetic metal oxide are mixed. The free ferromagnetic layer 10 having such a structure has a high MR ratio by directly contacting the metal ferromagnetic layer 10a formed of a material having high spin polarizability and thermal stability with the tunnel insulating layer 9. Realize. Further, the magnetically soft composite magnetic layer 10b causes exchange interaction to the metal ferromagnetic layer 10a to soften the metal ferromagnetic layer 10a and soften the entire free ferromagnetic layer 10.

  In order to obtain a softer free ferromagnetic layer than the free ferromagnetic layer 10 having the structure shown in FIG. 10, a ferromagnetic material containing Ni, typically as shown in FIG. The soft ferromagnetic layer 10c made of NiFe can be formed on the composite magnetic layer 10b. The composite magnetic layer 10b formed of the composite thin film described above has an effect of preventing diffusion even for Ni. Accordingly, diffusion of Ni contained in the soft ferromagnetic layer 10c into the tunnel insulating layer 9 is prevented by the composite magnetic layer 10b. Furthermore, the diffusion of Ni from the soft ferromagnetic layer 10 c to the upper wiring 3 is prevented by the oxide layer 14 described above. The structure shown in FIG. 11 is preferable in that diffusion of Ni to the tunnel insulating layer 9 and the upper electrode layer 3 can be prevented while increasing the MR ratio.

In the magnetoresistive element 4 shown in FIG. 10 or FIG. 11, since the composite magnetic layer 10b is added to the free ferromagnetic layer 10, the saturation magnetization of the free ferromagnetic layer 10 as a whole increases and the demagnetizing field increases. Have a problem. As a method of reducing the demagnetizing field, as shown in FIG. 12, the free ferromagnetic layer 10 is made of a metal ferromagnetic layer 10d, a composite magnetic layer 10e, a nonmagnetic layer 10f, a composite magnetic layer 10g, and a soft ferromagnetic layer. It is preferable to form the layer 10h. The metallic ferromagnetic layer 10d is formed of a ferromagnetic material having a high spin polarizability, such as CoFe. The composite magnetic layers 10e and 10g are formed of the composite thin film described above and exhibit soft ferromagnetism. The soft ferromagnetic layer 10h is formed of a ferromagnetic material including Ni, typically NiFe. The nonmagnetic layer 10f is formed of a material having a property of strongly coupling the composite magnetic layer 10e and the composite magnetic layer 10g antiferromagnetically, typically Cu, Cr, Rh, Ru, RuO _x .

  Due to the action of the nonmagnetic layer 10f, the spontaneous magnetization possessed by the metal ferromagnetic layer 10d and the composite magnetic layer 10e and the spontaneous magnetization possessed by the composite magnetic layer 10g and the soft ferromagnetic layer 10h are antiferromagnetically coupled. Become parallel. As disclosed in US Pat. No. 5,408,377, a free ferromagnetic layer 4 is sandwiched between two ferromagnetic layers and the ferromagnetic layer, and the ferromagnetic layer is antiferromagnetic. By configuring with a non-magnetic layer that is physically coupled, it is possible to reduce the effective saturation magnetic field and thus the demagnetizing field of the free ferromagnetic layer. By reducing the demagnetizing field, the reversal magnetic field (coercivity) of the free ferromagnetic layer 4 is reduced. Therefore, the structure shown in FIG. 12 can increase the MR ratio, prevent Ni from diffusing into the tunnel insulating layer 9 and the upper electrode layer 3, and further soften the free ferromagnetic layer 10. is there.

  In the structure of FIG. 12, if a sufficiently soft free ferromagnetic layer 10 is obtained, the soft ferromagnetic layer 10h may not be provided. By excluding the soft ferromagnetic layer 10h, Ni can be excluded from the free ferromagnetic layer 10 and the adverse effects of Ni diffusion can be fundamentally avoided.

Furthermore, in the structure of FIG. 12, in order to strengthen the antiferromagnetic coupling inside the free ferromagnetic layer 10, as shown in FIG. 13, at the interfaces on both sides of the nonmagnetic layer 10f, It is preferable to provide the metal ferromagnetic layer 10i. By using a ferromagnetic alloy containing Co as a main component and not containing Ni as the metal ferromagnetic layer 10i, a strong antiferromagnetic coupling can be obtained. It is preferable to provide the metal ferromagnetic layer 10i. The metal ferromagnetic layer 10 i is preferably formed of Co or Co ₉₀ Fe ₁₀ .

  In order to improve the characteristics of the magnetoresistive element, a magnetic bias layer 20 for applying an appropriate bias magnetic field to the free ferromagnetic layer 10 may be provided as shown in FIG. Application of an appropriate bias magnetic field eliminates asymmetry of the hysteresis curve of the free ferromagnetic layer 10 with respect to the magnetic field application direction.

  The magnetic bias layer 20 includes a protective layer 16, a ferromagnetic layer 17 formed of a ferromagnetic material, an antiferromagnetic layer 18 formed of an antiferromagnetic material, and a protective layer 19. The protective layers 16 and 19 are typically made of Ta and Zr. The ferromagnetic layer 17 is typically made of CoFe. The antiferromagnetic layer 18 is formed of an antiferromagnetic material containing Mn, typically PtMn or IrMn. The positions of the ferromagnetic layer 17 and the antiferromagnetic layer 18 are interchangeable.

  The antiferromagnetic layer 18 causes the problem of Mn diffusion, like the antiferromagnetic material 7. In order to prevent the problem of Mn diffusion, when the magnetic bias layer 20 is provided in the magnetoresistive element, the magnetic bias layer 20 is formed on the oxide layer 14, and the magnetic bias layer 20 and the upper contact layer are formed. 15, an oxide layer 21 is provided. The oxide layer 14 prevents Mn from diffusing into the tunnel insulating layer 9, and the oxide layer 21 prevents Mn from diffusing into the upper wiring 3.

  The physical properties required for the oxide layer 21 are the same as those of the oxide layer 13, and materials and structures suitable for the oxide layer 13 are also suitable for the oxide layer 21. First, in order to refine the structure of the oxide layer 21, maintain thermal stability up to a high temperature, and increase the effect of suppressing interdiffusion, the oxide layer 21 is made of Al, Mg, Si, Hf. , Li, Ca, and Ti are preferable. Furthermore, in order to improve the S / N ratio for detecting the resistance of the tunnel insulating layer 9, the resistance in the thickness direction of the oxide layer 21 is preferably smaller than the resistance in the thickness direction of the tunnel insulating layer 9. Furthermore, it is preferable that the thickness of the oxide layer 21 is 1 nm or less from the viewpoint of extremely reducing the resistance of the oxide layer 14 and improving the SN ratio of detection of the resistance of the tunnel insulating layer 9. Furthermore, forming the tunnel insulating layer 9 and the oxide layer 21 with the same material allows the tunnel insulating layer 9 and the oxide layer 21 to be formed with a common device and material, thereby reducing the cost required for manufacturing the MRAM. It is preferable in that it can be performed.

  The oxide layer 21 is formed by oxidation of a material having a higher ability to bond with oxygen than the material constituting the lower surface (the surface on the substrate 1 side) and the layer in contact with the upper surface (that is, the lower contact layer 12 and the seed layer 5). It is preferable to use a product. The use of an oxide of a material other than that for the oxide layer 21 is not preferable because oxygen diffuses in the layers in contact with the lower surface and the upper surface of the oxide layer 21 and the diffusion preventing effect of the oxide layer 21 is lost. When Ta is used for the upper contact layer 15 and Ta or Cr is used for the protective layer 19, oxides of Al, Mg, Si, Hf, Li, Ca, and Ti, which have lower free energy for oxide formation than those, are oxidized. It is preferable to use it as the physical layer 21.

  The structure of the lower contact layer 12 ′ in FIG. 2, the structure of the upper contact layer 15 ′ in FIG. 3, the structure of the fixed ferromagnetic layer 8 in FIG. 5, the structure of the free ferromagnetic layer 10 in FIGS. The structure of the magnetic bias layer 20 can be used in combination. For example, the structure shown in FIG. 15 can be adopted. In the MRAM of FIG. 15, the fixed ferromagnetic layer 8 is formed of a composite magnetic layer 8a and a ferromagnetic layer 8b. Furthermore, the free ferromagnetic layer 10 is formed of a metal ferromagnetic layer 10a, a composite ferromagnetic layer 10b, and a soft ferromagnetic layer 10c. Further, a magnetic bias layer 20 is provided on the oxide layer 14, and an oxide layer 21 is provided between the magnetic bias layer 20 and the upper contact layer 15.

  FIG. 16 shows a more realistic structure of the magnetoresistive element. The upper contact layer 15 and the upper wiring 3 are connected by a via 22. The via 22 is typically formed of Al, Cu, W, and TiN. Further, in the magnetoresistive element of FIG. 16, the tunnel insulating layer 9 is provided at a position where it does not overlap with the via 11 in the direction perpendicular to the main surface of the substrate 1. Such an arrangement of the tunnel insulating layer 9 is preferable in that defects of the tunnel insulating layer 9 can be reduced. It is difficult to avoid the formation of irregularities on the surface of the via 11 made of metal. As shown in FIG. 1, when the tunnel insulating layer 9 is located above the uneven via 11, the tunnel insulating layer 9 is also uneven, and defects are likely to occur in the tunnel insulating layer 9. As shown in FIG. 16, the arrangement in which the tunnel insulating layer 9 does not overlap the via 11 can reduce defects in the tunnel insulating layer 9.

  Even when the tunnel insulating layer 9 is arranged so as not to overlap with the via 11 in the direction perpendicular to the main surface of the substrate 1, the structure of the lower contact layer 12 'in FIG. 2 and the upper contact layer 15' in FIG. 5, the structure of the pinned ferromagnetic layer 8 in FIG. 5, the structure of the free ferromagnetic layer 10 in FIGS. 10 to 13, and the structure of the magnetic bias layer 20 in FIG. 14 can be used in combination. For example, the structure shown in FIG. 17 can be adopted. In the MRAM of FIG. 17, the magnetoresistive element 4 and the lower wiring 2 are electrically connected by a thick lower contact layer 12 '. Further, the fixed ferromagnetic layer 8 is formed of the composite magnetic layer 8a and the ferromagnetic layer 8b. A magnetic bias layer 20 is provided on the oxide layer 14, and an oxide layer 21 is provided between the magnetic bias layer 20 and the upper contact layer 15.

  The arrangement in which the tunnel insulating layer 9 does not overlap the via 11 in the direction perpendicular to the main surface of the substrate 1 is such that the lower wiring 2 electrically connected to the via 11 is connected to the via 11 as shown in FIG. This is suitable for the case where a write wiring 2 ′ is used exclusively for reading, electrically insulated from the lower wiring 2, and provided in parallel with the lower wiring 2. When such an arrangement is adopted, the voltage stored between the upper wiring 3 and the lower wiring 2 is applied to discriminate the data stored in the free ferromagnetic layer 10 (that is, the detection of the resistance of the tunnel insulating layer 9). This is done by detecting the current that flows. On the other hand, data is written to the free ferromagnetic layer 10 by passing a write current through the upper wiring 3 and the write wiring 2 '.

  Further, when a Cu layer is used as the upper wiring 3, as shown in FIG. 18, the oxide layer 14 interposed between the free ferromagnetic layer 10 and the upper wiring layer 3 is It is preferable that the upper wiring 3 and the interlayer insulating film (not labeled) covering the magnetoresistive element 4 are separated from each other. In this case, the upper contact layer 15 is not used, and the protective layer 23 is provided between the free ferromagnetic layer 10 and the oxide layer 14. The protective layer 23 is typically formed of Ta and Zr. In such a structure, prevention of mutual diffusion between the magnetoresistive element 4 and the upper wiring 3 and prevention of diffusion of Cu constituting the upper wiring 3 into the interlayer insulating film are realized by the oxide layer 14. This is preferable.

  It is obvious that the structure of the magnetoresistive element 4 of the present embodiment described above can be applied to a magnetic read head.

Improvement of the characteristics of the magnetoresistive element by providing the oxide layer 13 and the oxide layer 14 was confirmed by two samples having the following structures.
Sample of Comparative Example 1:
Substrate / Ta (3 nm) / AlCu (50 nm) / Ta (3 nm) / Ni ₈₁ Fe ₁₉ (3 nm) / IrMn (10 nm) / Co ₉₀ Fe ₁₀ (6 nm) / AlO _x (2 nm) / Co ₉₀ Fe ₁₀ (2 .5 nm) / Ni ₈₁ Fe ₁₉ (7.5 nm) / Ta (5 nm) / AlCu (300 nm)
Sample of Example 1 (invention):
Substrate / Ta (3 nm) / AlCu (50 nm) / Ta (3 nm) / Al ₂ O ₃ (1 nm) / Ta (3 nm) / Ni ₈₁ Fe ₁₉ (3 nm) / IrMn (10 nm) / Co ₉₀ Fe ₁₀ (6 nm) / AlO _x (2 nm) / Co ₉₀ Fe ₁₀ (2.5 nm) / Ni ₈₁ Fe ₁₉ (7.5 nm) / Al ₂ O ₃ (1 nm) / Ta (5 nm) / AlCu (300 nm)

The sample of Comparative Example 1 corresponds to a structure in which the oxide layer 13 and the oxide layer 14 are excluded from the structure of FIG. The sample of Example 1 has a structure corresponding to the structure of FIG. 1 and includes an oxide layer 13 and an oxide layer 14 formed of Al ₂ O ₃ having a thickness of 1 nm. The AlCu layer having a thickness of 50 nm corresponds to the lower wiring 2, and the AlCu layer having a thickness of 300 nm corresponds to the upper wiring 3.

FIG. 19 shows the dependence of the MR ratio of the samples of Comparative Example 1 and Example 1 on the heat treatment temperature. The MR ratio of the sample of Comparative Example 1 is deteriorated by heat treatment at a temperature exceeding about 300 ° C., whereas the sample of Example 1 can withstand heat treatment up to a higher temperature of 300 ° C. The degree of decrease is low. This is presumably because Al and Cu were prevented from diffusing from the AlCu layer to the magnetic tunnel junction by the oxide layer 13 and the oxide layer 14 formed of Al ₂ O ₃ having a thickness of 1 nm.

Furthermore, the effect of preventing the oxide layer 13 from diffusing Ni and Mn from the buffer layer 6 and the antiferromagnetic layer 7 to the lower wiring 2 was confirmed using three samples having the following structures.
Sample of Comparative Example 2:
Substrate / Ta (3 nm) / AlCu (50 nm) / Ta (3 nm) / Ni ₈₁ Fe ₁₉ (3 nm) / IrMn (10 nm) / Co ₉₀ Fe ₁₀ (4 nm)
Sample of Example 2 (invention):
Substrate / Ta (3 nm) / AlCu (50 nm) / Ta (3 nm) / Al ₂ O ₃ (1 nm) / Ta (3 nm) / Ni ₈₁ Fe ₁₉ (3 nm) / IrMn (10 nm) / Co ₉₀ Fe ₁₀ (4 nm)
Sample of Example 3 (invention):
Substrate / Ta (3 nm) / AlCu (50 nm) / Ta (3 nm) / MgO (1 nm) / Ta (3 nm) / Ni ₈₁ Fe ₁₉ (3 nm) / IrMn (10 nm) / Co ₉₀ Fe ₁₀ (4 nm)

The sample of Comparative Example 2 corresponds to a structure in which the oxide layer 13 is removed from a portion closer to the substrate 1 than the tunnel insulating layer 9 having the structure of FIG. The sample of Example 2 corresponds to a structure closer to the substrate 1 than the tunnel insulating layer 9, and 1 nm thick Al ₂ O ₃ is used as the oxide layer 13. The sample of Example 3 corresponds to a structure closer to the substrate 1 than the tunnel insulating layer 9, and MgO having a thickness of 1 nm is used as the oxide layer 13. For all samples, the AlCu layer with a thickness of 50 nm corresponds to the lower electrode 2.

FIG. 20 shows the dependence of the sheet resistance of the AlCu layer on the heat treatment temperature for the samples of Comparative Example 2 and Example 2. In the sample of Comparative Example 2 that does not have the oxide layer 13, when the heat treatment temperature exceeds 300 ° C., the sheet resistance of the AlCu layer increases rapidly. On the other hand, the sample of Example 2 having the oxide layer 13 formed of Al ₂ O ₃ does not increase the sheet resistance even when heat treatment at 350 ° C., and the sheet resistance does not increase even when heat treatment at about 400 ° C. is performed. The increase is slight.

FIG. 21 shows the sheet resistance of the AlCu layer of the samples of Comparative Example 2, Example 2 and Example 3 when the heat treatment is performed at various temperatures. As shown in FIG. 21, the sheet resistance of the sample of Comparative Example 2 that does not have the oxide layer 13 is increased by heat treatment at 350 ° C. and 400 ° C. On the other hand, the sample of Example 2 having the oxide layer 13 formed of Al ₂ O ₃ and the sample of Example 3 having the oxide layer 13 formed of MgO are affected by the heat treatment at 350 ° C. Absent. Further, in the samples of Example 2 and Example 3, the increase in sheet resistance by heat treatment at 400 ° C. is slight.

Further, the effect of the oxide layer 14 preventing mutual diffusion between the upper wiring 3 and the free ferromagnetic layer 10 was confirmed using two samples having the following structures.
Sample of Comparative Example 3:
Substrate / Ta (1.5 nm) / Co ₉₀ Fe ₁₀ (10 nm) / AlO _x (2 nm) / Co ₉₀ Fe ₁₀ (2.5 nm) / Ni ₈₁ Fe ₁₉ (7.5 nm) / Ta (5 nm) / AlCu ( 300nm)
Sample of Example 4 (present invention)
Substrate / Ta (1.5 nm) / Co ₉₀ Fe ₁₀ (10 nm) / AlO _x (2 nm) / Co ₉₀ Fe ₁₀ (2.5 nm) / Ni ₈₁ Fe ₁₉ (7.5 nm) / Al ₂ O ₃ (1 nm) / Ta (5 nm) / AlCu (300 nm)

For both Comparative Example 3 and Example 4, the Co ₉₀ Fe ₁₀ layer with a thickness of 2.5 nm and the Ni ₈₁ Fe ₁₉ layer with a thickness of 7.5 nm correspond to the free ferromagnetic layer 10 and have a thickness of 300 nm. The AlCu layer corresponds to the upper wiring 3. The Al ₂ O ₃ layer having a thickness of 1 nm included in Example 4 corresponds to the oxide layer 14. In the samples of Comparative Example 3 and Example 4, the antiferromagnetic layer 7 is not provided in order to eliminate the influence of Mn diffusion.

FIG. 22 shows the dependence of the MR ratio of the samples of Comparative Example 3 and Example 4 on the heat treatment temperature. The sample of Comparative Example 3 deteriorates the MR ratio by heat treatment at a temperature exceeding about 300 ° C., whereas the sample of Example 4 can withstand heat treatment up to a higher temperature of 370 ° C. Further, the MR ratio is maintained at about 20% even in the heat treatment at 400 ° C. This is presumably because Al and Cu were prevented from diffusing from the AlCu layer to the magnetic tunnel junction by the oxide layer 14 formed of Al ₂ O ₃ having a thickness of 1 nm.

FIG. 23 shows the magnetization curve of the Ni ₈₁ Fe ₁₉ layer of the sample of Comparative Example 3, and FIG. 24 shows the magnetization curve of the Ni ₈₁ Fe ₁₉ layer of the sample of Example 4. The magnetization curve is measured by a vibration type magnetometer. As shown in FIG. 23, when the sample of Comparative Example 3 is heat-treated at 380 ° C., the saturation magnetization decreases, the steep hysteresis curve is lost, and the coercive force increases. This is due to interdiffusion between the AlCu layer and the Ni ₈₁ Fe ₁₉ layer. On the other hand, as shown in FIG. 24, the magnetization curve of the sample of Example 4 is not affected by the heat treatment at 380 ° C. This is because the Al ₂ O ₃ layer prevents interdiffusion between the AlCu layer and the Ni ₈₁ Fe ₁₉ layer.

Further, the following four samples confirmed that the oxide layer 14 formed of the Al ₂ O ₃ layer and the MgO layer prevents mutual diffusion between the upper wiring 3 and the free ferromagnetic layer 10. It was.
Sample of Comparative Example 4:
Substrate / Co ₉₀ Fe ₁₀ (2.5 nm) / Ni ₈₁ Fe ₁₉ (7.5 nm) / Ta (6 nm) / AlCu (300 nm)
Sample of Comparative Example 5:
Substrate / Co ₉₀ Fe ₁₀ (2.5 nm) / Ni ₈₁ Fe ₁₉ (7.5 nm) / Ta (50 nm) / AlCu (300 nm)
Sample of Example 5 (invention):
Substrate / Co ₉₀ Fe ₁₀ (2.5 nm) / Ni ₈₁ Fe ₁₉ (7.5 nm) / Al ₂ O ₃ (1 nm) / Ta (6 nm) / AlCu (300 nm)
Sample of Example 6 (invention):
Substrate / Co ₉₀ Fe ₁₀ (2.5 nm) / Ni ₈₁ Fe ₁₉ (7.5 nm) / MgO (1 nm) / Ta (6 nm) / AlCu (300 nm)

The sample of Comparative Example 4 corresponds to a structure in which the oxide layer 14 is removed from a portion farther from the substrate 1 than the tunnel insulating layer 9 having the structure of FIG. The sample of Comparative Example 5 has a structure in which Ta corresponding to the upper contact layer 15 ′ is thickened to 50 nm in the sample of Comparative Example 4 and diffusion from AlCu which is the upper wiring 3 is prevented. The sample of Example 5 corresponds to the structure of a portion further away from the substrate 1 than the tunnel insulating layer 9 having the structure of FIG. 1, and Al ₂ O ₃ having a thickness of 1 nm is used as the oxide layer 14. The sample of Example 6 corresponds to the structure of a portion farther from the substrate 1 than the tunnel insulating layer 9 having the structure of FIG. 1, and MgO having a thickness of 1 nm is used as the oxide layer 14. For all of Comparative Example 4, Comparative Example 5, Example 5, and Example 6, the laminated structure composed of the Co ₉₀ Fe ₁₀ layer and the Ni ₈₁ Fe ₁₉ layer corresponds to the free ferromagnetic layer.

FIG. 25 shows the dependence of the saturation magnetization of the free ferromagnetic layer of the samples of Comparative Example 4, Comparative Example 5, Example 5, and Example 6 on the heat treatment temperature. In the sample of Comparative Example 4, as shown in FIG. 25, the saturation magnetization is lowered and the coercive force is increased by the heat treatment at about 380 ° C. The saturation magnetization almost disappeared after the heat treatment at 400 ° C., and the magnetization curve showed a paramagnetic shape. In Comparative Example 5, the coercive force did not increase as in Comparative Example 4, but the saturation magnetization value steadily decreased with the temperature of the heat treatment temperature. Further, in Comparative Example 4, interdiffusion between the AlCu layer corresponding to the upper wiring and the Co ₉₀ Fe ₁₀ layer and Ni ₈₁ Fe ₁₉ layer corresponding to the free ferromagnetic layer 10 was significant. In Comparative Example 5, interdiffusion between the Al layer, the Co ₉₀ Fe ₁₀ layer, and the Ni ₈₁ Fe ₁₉ layer is prevented, but the 50 nm Ta layer corresponding to the upper contact layer 15 ′, the Al layer, and the Co ₉₀ Fe layer Interdiffusion occurs between the ₁₀ layers and the Ni ₈₁ Fe ₁₉ layer. In contrast, the samples of Examples 5 and 6 have no change in saturation magnetization at all heat treatment temperatures, and can withstand heat treatment up to a higher temperature of 400 ° C.

Furthermore, the following sample confirmed that the oxide layer 14 formed of an AlO _x layer enables the free ferromagnetic layer 10 to be thinned to 3 nm or less.
Sample of Example 7 (present invention):
Substrate / Ta (10 nm) / AlO _x / Ni ₈₁ Fe ₁₉ / AlO _x / Ta (10 nm)
The AlO _x film closer to the substrate corresponds to the tunnel insulating layer 9, and the Ni ₈₁ Fe ₁₉ film corresponds to the free ferromagnetic layer 10. Further, the AlO _x film farther from the substrate corresponds to the oxide layer 14, and the Ta film farther from the substrate corresponds to the upper contact layer 15. The AlOx film corresponding to the tunnel insulating layer 9 is formed by oxidizing an Al film having a thickness of 1.5 nm by oxygen plasma, and the AlOx film corresponding to the oxide layer 14 has a thickness of 0.65 nm. The Al film is formed by being oxidized by oxygen plasma. The thickness of the Ni ₈₁ Fe ₁₉ film is selected from 3.0 nm, 2.6 nm, 2.2 nm, 1.4 nm, and 1.0 nm. The Ni ₈₁ Fe ₁₉ film is formed by sputtering. The heat treatment and the measurement of magnetization were performed under the same conditions as the sample of FIG. That is, the heat treatment temperature is 250 to 400 ° C., and the time is 30 minutes. The magnetization M _s is measured by a vibrating magnetometer.

As shown in FIG. 36, the sample of Example 7 shows a stable 4πM _s × t with respect to the heat treatment up to 400 ° C. even when the thickness of the Ni ₈₁ Fe ₁₉ film is reduced to 1.4 nm. ing. 4πM _s × t could be reduced from 2.2 (T · nm) to 1.2 (T · nm). The coercive force (reversal magnetic field) of the Ni ₈₁ Fe ₁₉ film of the sample of Example 7 was stable in the range of 0.5 to 1.5 (Oe). This means that the insertion of the oxide layer 14 makes the free ferromagnetic layer 10 thinner, thereby making it possible to sufficiently reduce the coercive force sufficiently stably.

Furthermore, the following sample confirmed the effect of reducing the product Ms × t by constituting the free ferromagnetic layer 10 with the ferromagnetic layer 10a and the diffusion layer 10b formed of a nonmagnetic metal.
Sample of Example 8 (invention):
Substrate / Ta (10 nm) / AlO _x / Ni ₈₁ Fe ₁₉ (1.6 nm) / Nonmagnetic metal layer / AlO _x / Ta (10 nm)
The AlO _x film closer to the substrate corresponds to the tunnel insulating layer 9. The Ni ₈₁ Fe ₁₉ film corresponds to the ferromagnetic layer 10 a of the free ferromagnetic layer 10, the nonmagnetic metal layer corresponds to the diffusion layer 10 b, and the AlO _x film far from the substrate corresponds to the oxide layer 14. The Ta film farther from the substrate corresponds to the upper contact layer 15. The AlOx film corresponding to the tunnel insulating layer 9 is formed by oxidizing an Al film having a thickness of 1.5 nm by oxygen plasma, and the AlOx film corresponding to the oxide layer 14 has a thickness of 0.65 nm. The Al film is formed by being oxidized by oxygen plasma. As the nonmagnetic metal layer, a Ta film having a thickness of 0.6 nm, a Ta film having a thickness of 0.3 nm, a Ru film having a thickness of 0.3 nm, and a Cu film having a thickness of 0.3 nm were used.

As shown in FIG. 37, the structure of the sample of Example 8 makes it possible to reduce 4πMs × t of the Ni ₈₁ Fe ₁₉ film to 1 (T · nm) or less. Moreover, the 4πMs × t of the Ni ₈₁ Fe ₁₉ film of the sample of Example 8 is stable with respect to the heat treatment at 400 ° C. The coercive force of the Ni ₈₁ Fe ₁₉ film of the sample of Example 8 is 1.5 (Oe) or less, and a sufficiently low coercive force is obtained.

Further, it was confirmed by the following sample that the product Ms × t can be reduced even if the nonmagnetic metal layer is inserted in the middle of the free ferromagnetic layer 10.
Sample of Example 9 (invention):
Substrate / Ta (10 nm) / AlO _x / Ni ₈₁ Fe ₁₉ (1.6 nm) / Nonmagnetic metal layer / Ni ₈₁ Fe ₁₉ (0.8 nm) / AlO _x / Ta (10 nm)
The AlO _x film closer to the substrate corresponds to the tunnel insulating layer 9, and the two Ni ₈₁ Fe ₁₉ films sandwiching the nonmagnetic metal layer correspond to the ferromagnetic layers 10 a and 10 c of the free ferromagnetic layer 10. . The nonmagnetic metal layer corresponds to the diffusion layer 10b. Further, the AlO _x film farther from the substrate corresponds to the oxide layer 14, and the Ta film farther from the substrate corresponds to the upper contact layer 15. The AlOx film corresponding to the tunnel insulating layer 9 is formed by oxidizing an Al film having a thickness of 1.5 nm by oxygen plasma, and the AlOx film corresponding to the oxide layer 14 has a thickness of 0.65 nm. The Al film is formed by being oxidized by oxygen plasma. A 0.3 nm Ta film is used as the nonmagnetic metal layer.

As shown in FIG. 37, 4πM _s × t of the free ferromagnetic layer 10 of the sample of Example 9 is reduced to 1 (T · nm) or less, and the 4πM _s × t is a heat treatment at 400 ° C. It was stable against. This indicates that the product M _s × t of the free ferromagnetic layer 10 can be stably reduced by optimally adjusting the material, thickness, and position of the nonmagnetic metal layer. ing.

Furthermore, it was confirmed by the following sample that the product Ms × t can be reduced by oxidizing a part of the free ferromagnetic layer 10.
Sample of Comparative Example 6:
Substrate / Ta (10 nm) / AlO _x / Ni ₈₁ Fe ₁₉ (2.2 nm) / AlO _x / Ta (10 nm)
Sample of Example 10 (invention):
Substrate / Ta (10 nm) / AlO _x / Ni ₈₁ Fe ₁₉ / NiFeO _x / AlO _x / Ta (10 nm)
The AlO _x film closer to the substrate corresponds to the tunnel insulating layer 9, and the Ni ₈₁ Fe ₁₉ film corresponds to the free ferromagnetic layer 10. Further, the AlO _x film farther from the substrate corresponds to the oxide layer 14, and the Ta film farther from the substrate corresponds to the upper contact layer 15. The AlOx film corresponding to the tunnel insulating layer 9 is formed by oxidizing an Al film having a thickness of 1.5 nm by oxygen plasma. The Ni ₈₁ Fe ₁₉ film, the NiFeO _x film, and the AlO _x film corresponding to the oxide layer 14 are formed by the following steps. First, a Ni ₈₁ Fe ₁₉ film having a thickness of 2.2 nm was formed on the AlO _x film corresponding to the tunnel insulating layer 9. An Al film having a thickness of 0.65 nm was formed on the Ni ₈₁ Fe ₁₉ film. After the formation of the Al film, the surface of the Al film was exposed to oxygen plasma. By exposure to oxygen plasma, the Al film and a part of the Ni ₈₁ Fe ₁₉ film were oxidized, and the NiFeO _x film and the AlO _x film corresponding to the oxide layer 14 were formed.

As shown in FIG. 37, 4πM _s × t of the free ferromagnetic layer 10 of the sample of Example 10 was lower than 4πM _s × t of the free ferromagnetic layer 10 of the sample of Comparative Example 6. This means that the product Ms × t can be reduced by oxidizing a part of the free ferromagnetic layer 10.

In order to demonstrate that the reversal magnetic field of the free ferromagnetic layer 10 having a size of the order of submicron can be reduced by reducing the product M _s × t, the magnetization evaluation of the dot pattern was performed. The magnetoresistive element was processed into an elliptical dot-shaped sample having a vertical and horizontal size of 0.5 × 1.0 μm ² , and the processed sample was evaluated. Processing into dots was performed using photolithography and ion milling. The laminated structure of the formed sample is as follows.
Sample of Comparative Example 7:
Substrate / Ta (30 nm) / Ni ₈₁ Fe ₁₉ (2 nm) / Ir ₂₀ Mn ₈₀ / Co ₉₀ Fe ₁₀ / AlO _x / Ni ₈₁ Fe ₁₉ (3 nm) / Ta (10 nm)
Sample of Example 11 (invention):
Substrate / Ta (30 nm) / Ni ₈₁ Fe ₁₉ (2 nm) / Ir ₂₀ Mn ₈₀ / Co ₉₀ Fe ₁₀ / AlO _x / Ni ₈₁ Fe ₁₉ (1.6 nm) / AlO _x / Ta (10 nm)
The AlO _x film closer to the substrate corresponds to the tunnel insulating layer 9, and the Ni ₈₁ Fe ₁₉ film formed on the AlO _x film corresponding to the tunnel insulating layer 9 corresponds to the free ferromagnetic layer 10. . Further, the AlO _x film farther from the substrate corresponds to the oxide layer 14, and the Ta film farther from the substrate corresponds to the upper contact layer 15. The AlOx film corresponding to the tunnel insulating layer 9 is formed by oxidizing an Al film having a thickness of 1.5 nm by oxygen plasma, and the AlOx film corresponding to the oxide layer 14 has a thickness of 0.65 nm. The Al film is formed by being oxidized by oxygen plasma. A 0.3 nm Ta film is used as the nonmagnetic metal layer. The film thickness of the free ferromagnetic layer 10 of the sample of Comparative Example 7 is 3.0 nm, which is the limit of thinning by the prior art. 4πM _s × t of the sample of Comparative Example 7 was 2.2 (T · nm), and 4πM _s × t of the sample of Example 11 was 1.5 (T · nm).

FIG. 38 shows the magnetization curves of the free ferromagnetic layers of the sample of Comparative Example 7 and the sample of Example 11. Note that in FIG. 38, the offset magnetic field is canceled so that the reversed magnetic fields can be compared. The reversal magnetic field of the free ferromagnetic layer of the sample of Example 11 was 10 (Oe), which was lower than the reversal magnetic field 18 (Oe) of the sample of Comparative Example 7. Thus, in the sample of Example 11 in which 4πM _s × t was reduced, the switching magnetic field was also reduced.

Further, it was verified by the sample of the magnetoresistive element having the following laminated structure that the insertion of the oxide layer 14 maintains the squareness of the magnetoresistance curve of the free ferromagnetic layer and suppresses the variation of the reversal magnetic field.
Sample of Comparative Example 8:
Substrate / Ta (30 nm) / NiFe (2 nm) / Ir ₂₀ Mn ₈₀ (10 nm) / Co ₉₀ Fe ₁₀ (1.5 nm) / AlO _x / NiFe (3 nm) / Ta (30 nm)
Sample of Comparative Example 9:
Substrate / Ta (30 nm) / NiFe (2 nm) / Ir ₂₀ Mn ₈₀ (10 nm) / Co ₉₀ Fe ₁₀ (1.5 nm) / AlO _x / NiFe (4 nm) / Ta (30 nm)
Sample of Example 12 (invention):
Substrate / Ta (30 nm) / NiFe (2 nm) / Ir ₂₀ Mn ₈₀ (10 nm) / Co ₉₀ Fe ₁₀ (1.5 nm) / AlO _x / NiFe (2 nm) / AlO _x / Ta (30 nm)
The AlO _x film closer to the substrate corresponds to the tunnel insulating layer 9, and the Ni ₈₁ Fe ₁₉ film formed on the AlO _x film corresponding to the tunnel insulating layer 9 corresponds to the free ferromagnetic layer 10. . The NiFe films of the samples of Comparative Example 8, Comparative Example 9, and Example 12 all have a Ni composition ratio less than 82%, and therefore the magnetostriction constant of the NiFe film is positive. However, the composition of the NiFe film is different and therefore the magnetostriction constant is also different. The magnetostriction constants λ of the free ferromagnetic layers 10 of the samples of Comparative Example 8, Comparative Example 9, and Example 12 are about 2 × 10 ⁻⁵ , about 9 × 10 ⁻⁷ , and about 5 × 10 ⁻⁶ , respectively. .

The magnetoresistive element was formed by the following process. A word line made of AlCu wiring (corresponding to the lower wiring 2) was formed, and an SiO ₂ film was formed as an interlayer insulating film around the word line. Furthermore, after the SiO ₂ film was planarized by CMP (chemical mechanical polishing), a laminated film constituting a magnetoresistive element was formed on the planarized SiO ₂ film. The laminated film was formed by magnetron sputtering. Each material was formed under the conditions that the discharge argon pressure was 0.2 Pa or less and the film formation rate was about 0.2 nm / s. The tunnel barrier was formed by plasma oxidation after sputtering of the Al layer. The laminated film was heat-treated in a magnetic field of 5 kOe at 275 ° C. and then processed into a 0.7 × 1.4 μm oval shape by photolithography and ion milling to form a magnetoresistive element. The aspect ratio of the magnetoresistive element is 2. The junction surface where the free ferromagnetic layer and the tunnel insulating layer of the magnetoresistive element are joined is located above the word line. The magnetoresistive element and the word line are not directly connected. The distance between the word line and the junction surface of the magnetoresistive element is about 200 nm. The width of the word line is 1 μm. The major axis direction of this magnetoresistive element is aligned with the word line direction, and the crystalline magnetic anisotropy of the magnetoresistive element is aligned with the easy axis in the major axis direction by heat treatment in the magnetic field and sputtering in the magnetic field. . The width of the word line is about the same as the width of the magnetoresistive element in the short side direction. Further, an SiO ₂ film is formed as an interlayer insulating film around the magnetoresistive element. Vias are formed at a position directly above the magnetoresistive elements of the formed SiO ₂ film, AlCu alloy is deposited on the entire surface over the SiO ₂ film. Thereafter, the AlCu alloy was processed by photolithography and etching to form a bit line (corresponding to the upper wiring 3). The direction of the bit line and the word line is vertical. A strong compressive stress is generated in the short axis direction of the magnetoresistive element by the magnetic word line. Since the magnetostriction constant of the free ferromagnetic layer of the formed magnetoresistive element is positive, stress-induced magnetic anisotropy occurs in the major axis direction of the magnetoresistive element. Therefore, crystal magnetic anisotropy, shape magnetic anisotropy, and stress-induced magnetic anisotropy are almost aligned, and the magnetoresistive element has uniaxial magnetic anisotropy with its long axis as the easy axis. have.

Ten samples of the magnetoresistive elements of Comparative Examples 8 and 9 and Example 12 were randomly selected, and the magnetoresistance curves of the free ferromagnetic layers of the selected samples were measured. 39A to 39C show the measured magnetoresistance curves. As can be seen from FIG. 39A, the free ferromagnetic layer of the magnetoresistive element of Comparative Example 8 has a small 4πM _s × t of 2.2 (T · nm). That is, although the shape magnetic anisotropy is small, the switching magnetic field of the free ferromagnetic layer of Comparative Example 8 is the largest at 21 (Oe). This is thought to be because the reversal magnetic field increased due to the increase in stress-induced magnetic anisotropy due to the large magnetostriction constant. Further, the squareness of the magnetoresistance curve is good, but the variation is large. This indicates that the thickness of the free ferromagnetic layer is 3 nm, which is close to the limit of thinning, so that the mutual diffusion between the Ta film and the NiFe film is large and the magnetostriction constant varies due to the composition shift. ing.

As shown in FIG. 39B, the magnetoresistive element of Comparative Example 9 has the smallest reversal magnetic field of 13 (Oe), but the squareness of the magnetoresistive curve is poor and the loop lies down. There are also many Barkhausen noises appearing in the magnetoresistance curve. This is because the magnetostriction constant is very small and there is almost no stress-induced anisotropy, so the uniaxial anisotropy is weakened and the reversal magnetic field is reduced, and 4πM _s × t of the free ferromagnetic layer is 3.2 (T · nm). This is considered to reflect the fact that the influence of the demagnetizing field is increased due to the large element aspect ratio of 2. In order to improve the squareness and variation of the magnetoresistive curve of the magnetoresistive element of Comparative Example 9, the aspect ratio must be increased to reduce the demagnetizing field and increase the shape anisotropy. This causes problems such as an increase in element size and an increase in switching magnetic field.

As shown in FIG. 39C, in the magnetoresistive element of Example 12, the reversal field of the free ferromagnetic layer is as small as 16 (Oe), the squareness of the magnetoresistance curve is good, and the variation of the reversal field is also large. Smallest and excellent. This is because the magnetoresistive element of the present invention suppresses variations in the magnetostriction constant and appropriately induces stable stress-induced magnetic anisotropy, and the 4πM _s × t of the free ferromagnetic layer is about 2 (T · nm), the demagnetizing field is small and the shape magnetic anisotropy is also small.

  The following experiment proved that the magnetoresistive element of the present invention in which the stress-induced magnetic anisotropy is appropriately controlled can realize both a small aspect ratio and stabilization of characteristics. The magnetoresistive elements of Comparative Example 9 and Example 12 having various aspect ratios were formed. The short axis length of the magnetoresistive element was 0.5 or 0.7 μm, and the aspect ratio was 1.0 to 3.5. The sample of Example 12 having a large magnetostriction constant has a relatively large ratio K2 / K3 of the stress-induced magnetic anisotropy to the shape magnetic anisotropy. 6 or more. Of all the samples of Example 12, the sample having an aspect ratio of 1.6 or less has a stress-induced magnetic anisotropy larger than the shape magnetic anisotropy, that is, the ratio K2 / K3 is 1.0 or more. Please note that. Thirty magnetoresistive elements were formed for each condition, and the magnetoresistive curves of the formed magnetoresistive elements were evaluated. Furthermore, the yield of the magnetoresistive element was calculated. In calculating the yield, a magnetoresistive element having an abnormality in a magnetoresistive curve, for example, a sample in which Barkhausen noise is seen in the magnetoresistive curve, and a sample in which the loop shape is lying are defined as defective samples. It was done.

  FIG. 40 is a graph showing the yield of magnetoresistive elements. The magnetoresistive element of Comparative Example 9 having a small magnetostriction constant (that is, a small stress-induced magnetic anisotropy) shows a sufficiently large yield only for a sample having an aspect ratio of 3.5. This means that the sample of Comparative Example 9 needs to stabilize the characteristics due to shape magnetic anisotropy. On the other hand, the magnetoresistive element of Example 12 to which a sufficiently large stress-induced magnetic anisotropy is given exhibits a sufficient yield over the entire aspect ratio range of 1.0 to 3.5. This shows that a sufficiently large yield can be obtained even in an aspect ratio range of 2 or less by making the stress-induced magnetic anisotropy larger than the shape magnetic anisotropy.

FIG. 1 is a sectional view showing an embodiment of a magnetoresistive device according to the present invention. FIG. 2 is a cross-sectional view showing a first modification of the magnetoresistive device of the present embodiment including a thick lower contact layer 12 '. FIG. 3 is a cross-sectional view showing a second modification of the magnetoresistive device of the present embodiment including the thick upper contact layer 15 ′. FIG. 4 is a cross-sectional view showing a third modification of the magnetoresistive device of the present embodiment in which the antiferromagnetic layer 7 ′, the buffer layer 6 ′, and the seed layer 5 ′ constitute a part of the wiring. FIG. 5 is a cross-sectional view showing a fourth modification of the magnetoresistive device according to the present embodiment, which includes the fixed ferromagnetic layer 8 including the composite magnetic layer 8a. 6A and 6B are cross-sectional views showing the structure of the composite magnetic layer 8a. FIG. 7 is a graph showing the resistivity of a thin film formed by sputtering an alloy target formed of CoFe as a ferromagnetic material and Ta as a nonmagnetic metal with a sputtering gas containing oxygen gas. FIG. 8 is a graph showing the saturation magnetization of such a thin film. FIG. 9 shows a Co _2p spectrum obtained by performing XPS on such a thin film. FIG. 10 is a cross-sectional view showing a fifth modification of the magnetoresistive device of the present embodiment provided with the free ferromagnetic layer 10 including the composite magnetic layer 10b. FIG. 11 is a cross-sectional view showing a sixth modification of the magnetoresistive device of the present embodiment provided with a free ferromagnetic layer 10 including a composite magnetic layer 10b and a soft ferromagnetic layer 10c. FIG. 12 shows the magnetoresistive device according to the present embodiment, which includes a free ferromagnetic layer 10 including a metal ferromagnetic layer 10d, a composite magnetic layer 10e, a nonmagnetic layer 10f, a composite magnetic layer 10g, and a soft ferromagnetic layer 10h. It is sectional drawing which shows a 7th modification. FIG. 13 is a cross-sectional view showing a seventh modification of the magnetoresistive device of the present embodiment provided with a free ferromagnetic layer 10 including a metal ferromagnetic layer 10i. FIG. 14 is a cross-sectional view showing an eighth modified example of the magnetoresistive device of the present embodiment including the magnetic bias layer 20. FIG. 15 shows a ninth example of the magnetoresistive device according to the present embodiment including the composite magnetic layer 8a, the composite ferromagnetic layer 10b, the soft ferromagnetic layer 10c, the magnetic bias layer 20, and the oxide layer 21. It is sectional drawing which shows a modification. FIG. 16 is a cross-sectional view showing a tenth modification of the magnetoresistive device of the present embodiment having a structure in which the via 11 and the tunnel ferromagnetic layer 9 do not overlap in the direction perpendicular to the main surface of the substrate 1. is there. FIG. 17 is a cross-sectional view showing an eleventh modification of the magnetoresistive device of the present embodiment having a structure in which the via 11 and the tunnel ferromagnetic layer 9 do not overlap in the direction perpendicular to the main surface of the substrate 1. is there. FIG. 18 is a cross-sectional view showing a twelfth modification of the magnetoresistive device of this embodiment, which includes a write wiring 2 ′ separately from the lower wiring 2 used for reading. FIG. 19 is a diagram showing the dependence of the MR ratio of the magnetic tunnel junction on the heat treatment temperature for the samples of Comparative Example 1 and Example 1. FIG. 20 is a diagram showing the dependence of the sheet resistance of the AlCu layer on the heat treatment temperature for the samples of Comparative Example 2 and Example 2. FIG. 21 is a table showing the dependence of the sheet resistance of the AlCu layer on the heat treatment temperature for the samples of Comparative Example 2, Example 2, and Example 3. FIG. 22 is a diagram showing the dependence of the MR ratio of the magnetic tunnel junction on the heat treatment temperature for the samples of Comparative Example 3 and Example 4. FIG. 23 is a diagram showing the magnetization curve of the sample of Comparative Example 3. FIG. 24 is a diagram showing the magnetization curve of the sample of Example 4. FIG. 25 is a table showing the dependence of the saturation magnetization of the free ferromagnetic layer on the heat treatment temperature for the samples of Comparative Example 4, Comparative Example 5, Example 5, and Example 6. FIG. 26 is a graph showing the effect of heat treatment on 4πM _s × t of a free ferromagnetic layer with a thin film thickness t. FIG. 27 is a cross-sectional view showing the structure of a free ferromagnetic layer that reduces the product 4πM _s × t. FIG. 28 is a cross-sectional view showing another structure of the free ferromagnetic layer for reducing the product 4πM _s × t. FIG. 29 is a cross-sectional view showing still another structure of the free ferromagnetic layer that reduces the product 4πM _s × t. FIG. 30 is a cross-sectional view showing still another structure of the free ferromagnetic layer for reducing the product 4πM _s × t. FIG. 31 is a cross-sectional view showing an MRAM structure in which the directions of easy axes of shape magnetic anisotropy and stress-induced magnetic anisotropy coincide. FIG. 32 is a plan view showing an MRAM structure in which the directions of easy axes of shape magnetic anisotropy and stress-induced magnetic anisotropy coincide with each other. FIG. 33 is a plan view showing the relationship among magnetocrystalline anisotropy, shape magnetic anisotropy, and stress-induced magnetic anisotropy. FIG. 34 is a plan view showing another MRAM structure in which the directions of shape magnetic anisotropy and stress-induced magnetic anisotropy coincide. FIG. 35 is a plan view showing a preferred relationship between shape magnetic anisotropy and stress-induced magnetic anisotropy. FIG. 36 is a graph showing the influence of the heat treatment of 4πM _s × t for the sample of Example 7. FIG. 37 is a graph showing the influence of heat treatment of 4πM _s × t for the samples of Comparative Example 6 and Examples 8, 9, and 10. FIG. 38 is a magnetization curve of the free ferromagnetic layer of the sample of Comparative Example 7 and the sample of Example 11. FIG. 39A is a magnetoresistance curve of the free ferromagnetic layer of the sample of Comparative Example 8. FIG. 39B is a magnetoresistance curve of the free ferromagnetic layer of the sample of Comparative Example 9. FIG. 39C is a magnetoresistance curve of the free ferromagnetic layer of the sample of Example 12. FIG. 40C is a graph showing the relationship between the aspect ratio and the yield of the samples of Comparative Example 9 and Example 12.

Explanation of symbols

1: Substrate 2: Lower wiring 3: Upper wiring 4: Magnetoresistive element 5, 5 ′: Seed layer 6, 6 ′: Buffer layer 7, 7 ′: Antiferromagnetic layer 8: Fixed ferromagnetic layer 8a: Composite magnetic layer 8b: Metal ferromagnetic layer 9: Tunnel insulating layer 10: Free ferromagnetic layer 10a, 10d, 10i: Metal ferromagnetic layer 10b, 10e, 10g: Composite magnetic layer 10c, 10h: Soft ferromagnetic layer 10f: Nonmagnetic layer 11 : Via 12, 12 ': Lower contact layer 13, 13': Oxide layer 14: Oxide layer 15, 15 ': Upper contact layer 16: Protective layer 17: Ferromagnetic layer 18: Antiferromagnetic layer 19: Protective layer 20: Magnetic bias layer 21: Oxide layer 22: Via

Claims (22)

A free ferromagnetic layer having reversible free spontaneous magnetization, a fixed ferromagnetic layer having fixed fixed spontaneous magnetization, and a tunnel insulating layer interposed between the free ferromagnetic layer and the fixed ferromagnetic layer A magnetoresistive element including:
A nonmagnetic conductor electrically connecting the magnetoresistive element to another element;
A diffusion preventing structure provided between the conductor and the magnetoresistive element;
With
The diffusion preventing structure is formed of an oxide of an element having a lower free energy for oxide formation than an element constituting a layer in contact with the upper surface and the lower surface.
A magnetoresistive device characterized by that . The diffusion preventing structure is made of a material selected from the group consisting of AlOx, MgOx, SiOx, TiOx, CaOx, LiOx, and HfOx.
The magnetoresistive device according to claim 1, A free ferromagnetic layer having reversible free spontaneous magnetization, a fixed ferromagnetic layer having fixed fixed spontaneous magnetization, and a tunnel insulating layer interposed between the free ferromagnetic layer and the fixed ferromagnetic layer A magnetoresistive element including:
A nonmagnetic conductor electrically connecting the magnetoresistive element to another element;
A diffusion preventing structure provided between the conductor and the magnetoresistive element;
With
The diffusion prevention structure is formed of an oxynitride of an element having lower oxide formation free energy and nitride formation free energy than an element constituting a layer in contact with the upper and lower surfaces
A magnetoresistive device characterized by that . The diffusion prevention structure is formed of a material selected from the group consisting of AlNO, SiNO, TiNO, and HfNO.
The magnetoresistive device according to claim 3, wherein A free ferromagnetic layer having reversible free spontaneous magnetization, a fixed ferromagnetic layer having fixed fixed spontaneous magnetization, and a tunnel insulating layer interposed between the free ferromagnetic layer and the fixed ferromagnetic layer A magnetoresistive element including:
A nonmagnetic conductor electrically connecting the magnetoresistive element to another element;
A diffusion preventing structure provided between the conductor and the magnetoresistive element;
With
The diffusion prevention structure is an oxide layer and is formed of the same material as the tunnel insulating layer.
A magnetoresistive device characterized by that . The diffusion prevention structure is thinner than the tunnel insulating layer
The magnetoresistive device according to any one of claims 1, 2, and 5, characterized in that: A free ferromagnetic layer having reversible free spontaneous magnetization, a fixed ferromagnetic layer having fixed fixed spontaneous magnetization, and a tunnel insulating layer interposed between the free ferromagnetic layer and the fixed ferromagnetic layer A magnetoresistive element including:
A nonmagnetic conductor electrically connecting the magnetoresistive element to another element;
A diffusion preventing structure provided between the conductor and the magnetoresistive element;
With
The diffusion prevention structure is formed of a material selected from the group consisting of AlN, SiN, BN, and ZrN.
A magnetoresistive device characterized by that . The diffusion preventing structure has a function of preventing at least one of materials constituting the conductor from diffusing into the magnetoresistive element.
A magnetoresistive device according to any one of claims 1 to 7, characterized in that The diffusion preventing structure has a function of preventing at least one of the materials constituting the magnetoresistive element from diffusing into the conductor.
A magnetoresistive device according to any one of claims 1 to 7, characterized in that The conductor includes at least one element selected from the group consisting of Al, Cu, Ta, Ru, Zr, Ti, Mo, and W.
The magnetoresistive device according to any one of claims 1 to 9, wherein the magnetoresistive device is characterized in that The resistance in the thickness direction of the diffusion preventing structure is lower than the resistance in the thickness direction of the tunnel insulating layer.
The magnetoresistive device according to claim 1, wherein the magnetoresistive device is a magnetic resistance device. The diffusion preventing structure is a film having a thickness of 5 nm or less.
The magnetoresistive device according to any one of claims 1 to 11, wherein the magnetoresistive device is characterized in that A free ferromagnetic layer having reversible free spontaneous magnetization, a fixed ferromagnetic layer having fixed fixed spontaneous magnetization, and a tunnel insulating layer interposed between the free ferromagnetic layer and the fixed ferromagnetic layer A magnetoresistive element including:
A nonmagnetic conductor electrically connecting the magnetoresistive element to another element;
A diffusion preventing structure provided between the conductor and the magnetoresistive element;
With
The conductor includes a first conductor electrically connected to the fixed ferromagnetic layer without the tunnel insulating layer;
A second conductor electrically connected to the free ferromagnetic layer without passing through the tunnel insulating layer;
Including
The diffusion prevention structure includes a first diffusion prevention layer interposed between the first conductor and the fixed ferromagnetic layer;
A second diffusion preventing layer interposed between the second conductor and the free ferromagnetic layer;
Only including,
The first diffusion barrier layer is formed of a material selected from the group consisting of oxides and oxynitrides,
The second diffusion barrier layer is formed of a material selected from the group consisting of oxides and oxynitrides.
This is a magnetoresistive device. A free ferromagnetic layer having reversible free spontaneous magnetization, a fixed ferromagnetic layer having fixed fixed spontaneous magnetization, and a tunnel insulating layer interposed between the free ferromagnetic layer and the fixed ferromagnetic layer A magnetoresistive element including:
A nonmagnetic conductor electrically connecting the magnetoresistive element to another element;
A diffusion preventing structure provided between the conductor and the magnetoresistive element;
With
The conductor includes a first conductor electrically connected to the fixed ferromagnetic layer without the tunnel insulating layer;
A second conductor electrically connected to the free ferromagnetic layer without passing through the tunnel insulating layer;
Including
The diffusion prevention structure includes a first diffusion prevention layer interposed between the first conductor and the fixed ferromagnetic layer;
A second diffusion preventing layer interposed between the second conductor and the free ferromagnetic layer;
Including
The first diffusion prevention layer is an oxide of an element having an oxide generation energy lower than that of an element constituting a layer in contact with the upper surface and the lower surface thereof, or an oxide generation than an element constituting a layer in contact with the upper surface and the lower surface of the first diffusion prevention layer. Formed of either oxynitrides of elements with low free energy and low free energy of nitride formation,
The second diffusion prevention layer is an oxide of an element having an oxide generation energy lower than that of an element constituting a layer in contact with the upper surface and the lower surface thereof, or an oxide generation than an element constituting a layer in contact with the upper surface and the lower surface of the second diffusion prevention layer. Formed with either oxynitrides of elements with low free energy and low free energy for nitride formation
This is a magnetoresistive device. A free ferromagnetic layer having reversible free spontaneous magnetization, a fixed ferromagnetic layer having fixed fixed spontaneous magnetization, and a tunnel insulating layer interposed between the free ferromagnetic layer and the fixed ferromagnetic layer A magnetoresistive element including:
A nonmagnetic conductor electrically connecting the magnetoresistive element to another element;
A diffusion preventing structure provided between the conductor and the magnetoresistive element;
With
The conductor includes a first conductor electrically connected to the fixed ferromagnetic layer without the tunnel insulating layer;
A second conductor electrically connected to the free ferromagnetic layer without passing through the tunnel insulating layer;
Including
The diffusion prevention structure includes a first diffusion prevention layer interposed between the first conductor and the fixed ferromagnetic layer;
A second diffusion preventing layer interposed between the second conductor and the free ferromagnetic layer;
Including
The first diffusion barrier layer is formed of a material selected from the group consisting of AlN, SiN, BN, and ZrN,
The second diffusion barrier layer is formed of a material selected from the group consisting of AlN, SiN, BN, and ZrN.
This is a magnetoresistive device. A free ferromagnetic layer having reversible free spontaneous magnetization, a fixed ferromagnetic layer having fixed fixed spontaneous magnetization, and a tunnel insulating layer interposed between the free ferromagnetic layer and the fixed ferromagnetic layer A magnetoresistive element including:
A nonmagnetic conductor electrically connecting the magnetoresistive element to another element;
A diffusion preventing structure provided between the conductor and the magnetoresistive element;
With
The conductor includes a first conductor electrically connected to the fixed ferromagnetic layer without the tunnel insulating layer;
The diffusion prevention structure includes a first diffusion prevention layer interposed between the fixed ferromagnetic layer and the first conductor,
The magnetoresistive element further includes an antiferromagnetic layer bonded to the pinned ferromagnetic layer and containing Mn,
The antiferromagnetic layer is located between the pinned ferromagnetic layer and the diffusion barrier layer;
The first diffusion prevention layer is formed of a material selected from the group consisting of oxides and oxynitrides.
This is a magnetoresistive device. The free ferromagnetic layer includes a Ni-containing ferromagnetic film containing Ni,
The diffusion preventing structure is provided between the antiferromagnetic layer containing Mn and the conductor, or between the Ni-containing ferromagnetic film and the conductor.
The magnetoresistive device according to claim 16, wherein A free ferromagnetic layer having reversible free spontaneous magnetization, a fixed ferromagnetic layer having fixed fixed spontaneous magnetization, and a tunnel insulating layer interposed between the free ferromagnetic layer and the fixed ferromagnetic layer A magnetoresistive element including:
A nonmagnetic conductor electrically connecting the magnetoresistive element to another element;
A diffusion preventing structure provided between the conductor and the magnetoresistive element;
With
The conductor includes a second conductor electrically connected to the free ferromagnetic layer without the tunnel insulating layer;
The diffusion prevention structure includes a second diffusion prevention layer interposed between the free ferromagnetic layer and the second conductor,
The second diffusion barrier layer is formed of a material selected from the group consisting of oxides and oxynitrides.
This is a magnetoresistive device. The free ferromagnetic layer includes a Ni-containing ferromagnetic film containing Ni,
The second diffusion preventing layer is in direct contact with the Ni-containing ferromagnetic film.
The magnetoresistive device according to claim 18, wherein: A free ferromagnetic layer having reversible free spontaneous magnetization, a fixed ferromagnetic layer having fixed fixed spontaneous magnetization, and a tunnel insulating layer interposed between the free ferromagnetic layer and the fixed ferromagnetic layer A magnetoresistive element including:
A nonmagnetic conductor electrically connecting the magnetoresistive element to another element;
A diffusion preventing structure provided between the conductor and the magnetoresistive element;
With
The conductor includes a second conductor electrically connected to the free ferromagnetic layer without the tunnel insulating layer;
The diffusion prevention structure includes a second diffusion prevention layer interposed between the free ferromagnetic layer and the second conductor,
The second diffusion barrier layer is directly bonded to the free ferromagnetic layer;
The film thickness of the free ferromagnetic layer is 3 nm or less
This is a magnetoresistive device. 21. The magnetoresistive device according to claim 20, wherein a product of a saturation magnetization and a film thickness of the free ferromagnetic layer is 3 (T · nm) or less. The conductor includes a second conductor electrically connected to the free ferromagnetic layer without the tunnel insulating layer;
The magnetoresistive element further includes a magnetic bias element that applies a bias magnetic field to the free ferromagnetic layer,
The magnetic bias element includes a magnetic bias ferromagnetic layer and a magnetic bias antiferromagnetic layer bonded to the magnetic bias ferromagnetic layer and containing Mn,
The diffusion prevention structure includes a third diffusion prevention structure interposed between the magnetic bias element and the free ferromagnetic layer;
A fourth diffusion preventing structure interposed between the magnetic bias element and the second conductor;
The magnetoresistive device according to claim 1, comprising:

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